diff --git a/backend/__tests__/integration/externalImportConcurrency.test.js b/backend/__tests__/integration/externalImportConcurrency.test.js
new file mode 100644
index 00000000..6bd72ab7
--- /dev/null
+++ b/backend/__tests__/integration/externalImportConcurrency.test.js
@@ -0,0 +1,205 @@
+/**
+ * Two overlapping external imports insert every file twice (#1162).
+ *
+ * The route checked for an existing external_relpath and then inserted, with
+ * an fs.stat and a `sharp().metadata()` read sitting in between. A reporter
+ * double-clicked a slow import of a 6012-file tree and got 8004 rows.
+ *
+ * Both halves of the fix are driven here through the real route:
+ *
+ *   - the in-flight guard, which turns the second click into a 409 instead of
+ *     a second full walk of the tree;
+ *   - convergence when the guard cannot help (another replica, another
+ *     process), which is the unique index from migration 186 firing and the
+ *     loop counting a skip rather than dying or duplicating.
+ *
+ * The second is exercised by inserting a competing row from inside the mocked
+ * `sharp().metadata()` call — literally inside the window the bug lived in.
+ */
+
+const fs = require('fs');
+const path = require('path');
+const os = require('os');
+const express = require('express');
+const request = require('supertest');
+
+describe('concurrent external imports (#1162)', () => {
+  let tmpDir; let db; let app; let mediaRoot;
+  // When set, the mocked sharp metadata read inserts this row first — the
+  // other run winning the race between our SELECT and our INSERT.
+  let stealDuringMetadata = null;
+  let thumbnailDelayMs = 0;
+
+  beforeAll(async () => {
+    tmpDir = await fs.promises.mkdtemp(path.join(os.tmpdir(), 'picpeak-extdup-'));
+    mediaRoot = path.join(tmpDir, 'media');
+    await fs.promises.mkdir(path.join(mediaRoot, 'nas', 'individual'), { recursive: true });
+    for (const name of ['a.jpg', 'b.jpg', 'c.jpg']) {
+      await fs.promises.writeFile(path.join(mediaRoot, 'nas', 'individual', name), 'not-a-real-jpeg');
+    }
+
+    process.env.NODE_ENV = 'test';
+    process.env.TEST_DATABASE_PATH = path.join(tmpDir, 'data', 'db.sqlite');
+    await fs.promises.mkdir(path.dirname(process.env.TEST_DATABASE_PATH), { recursive: true });
+    process.env.STORAGE_PATH = path.join(tmpDir, 'storage');
+    process.env.EXTERNAL_MEDIA_ROOT = mediaRoot;
+    process.env.JWT_SECRET = process.env.JWT_SECRET || 'extdup-secret';
+
+    jest.resetModules();
+
+    jest.doMock('../../src/middleware/auth', () => ({
+      adminAuth: (req, _res, next) => { req.admin = { id: 1, username: 'tester', roleName: 'admin' }; next(); },
+    }));
+    jest.doMock('../../src/middleware/permissions', () => ({
+      requirePermission: () => (_req, _res, next) => next(),
+    }));
+    jest.doMock('../../src/middleware/ownership', () => ({
+      requireEventOwnership: (_req, _res, next) => next(),
+    }));
+
+    // The window. In production this is a real decode of a NAS-hosted file —
+    // hundreds of milliseconds during which the row we just proved absent can
+    // appear. Standing in for the other run here makes that deterministic.
+    jest.doMock('sharp', () => () => ({
+      metadata: async () => {
+        if (stealDuringMetadata) {
+          const { db: liveDb } = require('../../src/database/db');
+          await liveDb('photos').insert(stealDuringMetadata);
+          stealDuringMetadata = null;
+        }
+        return { width: 100, height: 200 };
+      },
+    }));
+
+    jest.doMock('../../src/services/imageProcessor', () => ({
+      generateThumbnail: jest.fn(async () => {
+        if (thumbnailDelayMs) await new Promise((r) => setTimeout(r, thumbnailDelayMs));
+        return 'thumbnails/mock.jpg';
+      }),
+      ensureThumbnail: jest.fn(),
+    }));
+
+    jest.doMock('../../src/utils/logger', () => ({
+      debug: jest.fn(), info: jest.fn(), warn: jest.fn(), error: jest.fn(),
+    }));
+
+    ({ db } = await require('./helpers/crmDb').bootCrmDb());
+
+    app = express();
+    app.use(express.json());
+    app.use('/api/admin/external-media', require('../../src/routes/adminExternalMedia'));
+  }, 180000);
+
+  afterAll(async () => {
+    if (db) await db.destroy?.();
+    await fs.promises.rm(tmpDir, { recursive: true, force: true }).catch(() => {});
+  });
+
+  async function seedEvent() {
+    await db('photos').del();
+    await db('events').del();
+    stealDuringMetadata = null;
+    thumbnailDelayMs = 0;
+    const [e] = await db('events').insert({
+      slug: `extdup-${Math.random().toString(36).slice(2, 8)}`,
+      event_type: 'wedding',
+      event_name: 'extdup',
+      event_date: '2026-01-01',
+      host_email: 'h@example.com',
+      admin_email: 'a@example.com',
+      password_hash: 'x',
+      share_link: `extdup-${Math.random()}`,
+      expires_at: new Date().toISOString(),
+      source_mode: 'reference',
+    }).returning('id');
+    return typeof e === 'object' ? e.id : e;
+  }
+
+  const runImport = (eventId) => request(app)
+    .post(`/api/admin/external-media/events/${eventId}/import-external`)
+    .send({ external_path: 'nas', recursive: true });
+
+  async function relpathCounts(eventId) {
+    const rows = await db('photos').where({ event_id: eventId }).select('external_relpath');
+    const counts = new Map();
+    for (const r of rows) counts.set(r.external_relpath, (counts.get(r.external_relpath) || 0) + 1);
+    return counts;
+  }
+
+  it('rejects a second import while the first is still running', async () => {
+    const eventId = await seedEvent();
+    // Enough to keep the first request inside its loop while the second
+    // arrives — the "slow import looks hung, so I clicked again" case.
+    thumbnailDelayMs = 20;
+
+    const [first, second] = await Promise.all([runImport(eventId), runImport(eventId)]);
+
+    const statuses = [first.status, second.status].sort();
+    expect(statuses).toEqual([200, 409]);
+    const rejected = first.status === 409 ? first : second;
+    expect(rejected.body.error).toMatch(/already running/i);
+  });
+
+  it('leaves exactly one row per file after both runs', async () => {
+    const eventId = await seedEvent();
+    thumbnailDelayMs = 20;
+
+    await Promise.all([runImport(eventId), runImport(eventId)]);
+
+    const counts = await relpathCounts(eventId);
+    expect(counts.size).toBe(3);
+    expect([...counts.values()]).toEqual([1, 1, 1]);
+  });
+
+  it('releases the event once the import finishes, so a re-import still works', async () => {
+    const eventId = await seedEvent();
+
+    expect((await runImport(eventId)).status).toBe(200);
+    // Not 409 — the guard is per run, not a permanent lock on the event.
+    const second = await runImport(eventId);
+    expect(second.status).toBe(200);
+    expect(second.body.imported).toBe(0);
+    expect(second.body.skipped).toBe(3);
+  });
+
+  it('converges when another writer wins the race mid-file', async () => {
+    // The guard is in-process, so it cannot see a second replica. This is what
+    // the unique index is for: the insert bounces, and the file is counted as
+    // skipped rather than duplicated or lost to a 500.
+    const eventId = await seedEvent();
+    stealDuringMetadata = {
+      event_id: eventId,
+      filename: 'a.jpg',
+      path: 'x/a.jpg',
+      type: 'individual',
+      source_origin: 'external',
+      external_relpath: path.join('individual', 'a.jpg'),
+    };
+
+    const res = await runImport(eventId);
+
+    expect(res.status).toBe(200);
+    const counts = await relpathCounts(eventId);
+    expect(counts.get(path.join('individual', 'a.jpg'))).toBe(1);
+    // Two imported by us, one lost to the other writer and reported honestly.
+    expect(res.body.imported).toBe(2);
+    expect(res.body.skipped).toBe(1);
+  });
+
+  it('does not let one contended file abort the rest of the import', async () => {
+    const eventId = await seedEvent();
+    stealDuringMetadata = {
+      event_id: eventId,
+      filename: 'a.jpg',
+      path: 'x/a.jpg',
+      type: 'individual',
+      source_origin: 'external',
+      external_relpath: path.join('individual', 'a.jpg'),
+    };
+
+    await runImport(eventId);
+
+    // All three files present — the contended one via the other writer's row.
+    expect((await relpathCounts(eventId)).size).toBe(3);
+  });
+});
diff --git a/backend/__tests__/migrations/186_external_relpath_unique.test.js b/backend/__tests__/migrations/186_external_relpath_unique.test.js
new file mode 100644
index 00000000..76bc584f
--- /dev/null
+++ b/backend/__tests__/migrations/186_external_relpath_unique.test.js
@@ -0,0 +1,554 @@
+/**
+ * One row per external file per event (#1162).
+ *
+ * The migration has two halves and they fail differently: the cleanup can take
+ * out the wrong row of a pair (losing a thumbnail, orphaning an event's hero),
+ * and the index can fail to be created at all — leaving an install that looks
+ * migrated and is still racing. Both are pinned here.
+ */
+
+const path = require('path');
+const fs = require('fs');
+const os = require('os');
+
+const migration = require('../../migrations/core/186_external_relpath_unique');
+
+describe('migration 186 — unique (event_id, external_relpath) (#1162)', () => {
+  let knex; let tmpDir;
+
+  beforeAll(async () => {
+    tmpDir = await fs.promises.mkdtemp(path.join(os.tmpdir(), 'picpeak-mig186-'));
+    knex = require('knex')({
+      client: 'sqlite3',
+      connection: { filename: path.join(tmpDir, 'db.sqlite') },
+      useNullAsDefault: true,
+    });
+  });
+
+  afterAll(async () => {
+    if (knex) await knex.destroy();
+    await fs.promises.rm(tmpDir, { recursive: true, force: true }).catch(() => {});
+  });
+
+  beforeEach(async () => {
+    for (const table of [
+      'photos', 'events', 'photo_categories', 'photo_feedback',
+      'photo_admin_marks', 'photo_faces', 'image_access_logs', 'transfer_files',
+      'event_people', 'event_people_merge_dismissals',
+    ]) {
+      await knex.schema.dropTableIfExists(table);
+    }
+    await knex.schema.createTable('events', (t) => {
+      t.increments('id').primary();
+      t.integer('hero_photo_id');
+      t.string('download_zip_path');
+      t.string('download_zip_generated_at');
+    });
+    await knex.schema.createTable('photo_categories', (t) => {
+      t.increments('id').primary();
+      t.integer('hero_photo_id');
+    });
+    await knex.schema.createTable('photos', (t) => {
+      t.increments('id').primary();
+      t.integer('event_id');
+      t.string('external_relpath');
+      t.string('thumbnail_path');
+      t.string('source_origin').defaultTo('managed');
+      t.integer('feedback_count').defaultTo(0);
+      t.integer('like_count').defaultTo(0);
+      t.decimal('average_rating', 3, 2).defaultTo(0);
+      t.integer('favorite_count').defaultTo(0);
+      t.integer('reaction_count').defaultTo(0);
+      t.integer('color_label_count').defaultTo(0);
+      t.string('face_status');
+      t.integer('view_count').defaultTo(0);
+      t.integer('download_count').defaultTo(0);
+      t.integer('face_count');
+      t.string('face_started_at');
+      t.text('face_error');
+    });
+    // Declared exactly as the real schema declares them — CASCADE and all.
+    // The point of these tables here is that SQLite does NOT enforce any of
+    // it (PicPeak never sets `PRAGMA foreign_keys = ON`), so a bare delete of
+    // the photo row leaves every one of them dangling.
+    await knex.schema.createTable('photo_feedback', (t) => {
+      t.increments('id').primary();
+      t.integer('photo_id').references('id').inTable('photos').onDelete('CASCADE');
+      t.integer('event_id');
+      t.string('feedback_type');
+      t.text('comment_text');
+      t.string('guest_identifier');
+      // Per-person guest identity (migration 078). Nullable: galleries without
+      // guest identity leave it NULL and fall back to guest_identifier.
+      t.integer('guest_id');
+      t.integer('rating');
+      t.boolean('is_hidden').defaultTo(false);
+      t.boolean('is_approved').defaultTo(true);
+    });
+    await knex.schema.createTable('photo_admin_marks', (t) => {
+      t.increments('id').primary();
+      t.integer('photo_id').notNullable().references('id').inTable('photos').onDelete('CASCADE');
+      t.integer('event_id');
+      t.integer('admin_id');
+      t.integer('rating');
+      // Independently writable alongside rating, per photoAdminMarksService.
+      t.string('color_label', 16);
+      t.unique(['photo_id', 'admin_id'], 'photo_admin_marks_photo_admin_uniq');
+    });
+    await knex.schema.createTable('photo_faces', (t) => {
+      t.increments('id').primary();
+      t.integer('photo_id').references('id').inTable('photos').onDelete('CASCADE');
+      t.integer('event_id');
+      // purgePhotoFaces rebuilds the people that lose members, so the cluster
+      // link and the vectors recomputeCentroid reads have to be here for this
+      // to exercise the real path rather than a stub.
+      t.integer('person_id');
+      t.binary('embedding');
+      t.float('det_score');
+    });
+    await knex.schema.createTable('event_people', (t) => {
+      t.increments('id').primary();
+      t.integer('event_id');
+      t.binary('centroid');
+      t.integer('face_count').defaultTo(0);
+    });
+    await knex.schema.createTable('event_people_merge_dismissals', (t) => {
+      t.increments('id').primary();
+      t.integer('event_id');
+      t.binary('centroid_a');
+      t.binary('centroid_b');
+    });
+    await knex.schema.createTable('image_access_logs', (t) => {
+      t.increments('id').primary();
+      t.integer('photo_id');
+    });
+    await knex.schema.createTable('transfer_files', (t) => {
+      t.increments('id').primary();
+      t.integer('transfer_id');
+      t.integer('photo_id');
+      t.unique(['transfer_id', 'photo_id'], 'transfer_files_unique');
+    });
+  });
+
+  /** Two duplicate rows for the same file: id 1 survives, id 2 is doomed. */
+  const seedPair = async () => {
+    await knex('photos').insert([
+      { event_id: 1, external_relpath: 'a/x.jpg', thumbnail_path: 't', source_origin: 'external' },
+      { event_id: 1, external_relpath: 'a/x.jpg', thumbnail_path: 't', source_origin: 'external' },
+    ]);
+  };
+
+  const rows = () => knex('photos').orderBy('id', 'asc').select('*');
+
+  it('collapses a duplicated pair to one row and leaves distinct paths alone', async () => {
+    await knex('photos').insert([
+      { event_id: 1, external_relpath: 'a/x.jpg', thumbnail_path: 't1', source_origin: 'external' },
+      { event_id: 1, external_relpath: 'a/x.jpg', thumbnail_path: 't2', source_origin: 'external' },
+      { event_id: 1, external_relpath: 'a/y.jpg', thumbnail_path: 't3', source_origin: 'external' },
+    ]);
+
+    await migration.up(knex);
+
+    const after = await rows();
+    expect(after.map((r) => r.external_relpath)).toEqual(['a/x.jpg', 'a/y.jpg']);
+    // Lowest id survives when both sides are equally complete.
+    expect(after[0].id).toBe(1);
+  });
+
+  it('does not collapse the same path across different events', async () => {
+    // The constraint is per event. Two events referencing the same NAS folder
+    // is a supported setup, and treating those as duplicates would delete one
+    // event's entire library.
+    await knex('photos').insert([
+      { event_id: 1, external_relpath: 'a/x.jpg', source_origin: 'external' },
+      { event_id: 2, external_relpath: 'a/x.jpg', source_origin: 'external' },
+    ]);
+
+    await migration.up(knex);
+
+    expect(await knex('photos').count('* as c').first()).toEqual({ c: 2 });
+  });
+
+  it('never touches managed rows, however many carry NULL', async () => {
+    // Every managed photo has external_relpath NULL. Grouping on it without
+    // the NOT NULL filter would make them all one enormous "duplicate" group
+    // and delete the entire library bar one row.
+    await knex('photos').insert([
+      { event_id: 1, external_relpath: null, source_origin: 'managed' },
+      { event_id: 1, external_relpath: null, source_origin: 'managed' },
+      { event_id: 1, external_relpath: null, source_origin: 'managed' },
+    ]);
+
+    await migration.up(knex);
+
+    expect(await knex('photos').count('* as c').first()).toEqual({ c: 3 });
+  });
+
+  it('keeps the row that has a thumbnail, not merely the lowest id', async () => {
+    // An import killed mid-flight leaves rows without a thumbnail. Dropping
+    // the completed one would blank a tile in the grid for no reason.
+    await knex('photos').insert([
+      { event_id: 1, external_relpath: 'a/x.jpg', thumbnail_path: null, source_origin: 'external' },
+      { event_id: 1, external_relpath: 'a/x.jpg', thumbnail_path: 'thumb.jpg', source_origin: 'external' },
+    ]);
+
+    await migration.up(knex);
+
+    const after = await rows();
+    expect(after).toHaveLength(1);
+    expect(after[0].thumbnail_path).toBe('thumb.jpg');
+  });
+
+  it('repoints a hero that pointed at the row being removed', async () => {
+    // events.hero_photo_id is ON DELETE SET NULL, so without this the cleanup
+    // silently strips the event's hero image — a visible regression caused
+    // entirely by the fix.
+    await knex('photos').insert([
+      { event_id: 1, external_relpath: 'a/x.jpg', thumbnail_path: 't', source_origin: 'external' },
+      { event_id: 1, external_relpath: 'a/x.jpg', thumbnail_path: 't', source_origin: 'external' },
+    ]);
+    await knex('events').insert({ id: 1, hero_photo_id: 2 });
+    await knex('photo_categories').insert({ id: 1, hero_photo_id: 2 });
+
+    await migration.up(knex);
+
+    expect((await knex('events').where({ id: 1 }).first()).hero_photo_id).toBe(1);
+    expect((await knex('photo_categories').where({ id: 1 }).first()).hero_photo_id).toBe(1);
+  });
+
+  it('leaves a hero that pointed at the survivor untouched', async () => {
+    await knex('photos').insert([
+      { event_id: 1, external_relpath: 'a/x.jpg', thumbnail_path: 't', source_origin: 'external' },
+      { event_id: 1, external_relpath: 'a/x.jpg', thumbnail_path: 't', source_origin: 'external' },
+    ]);
+    await knex('events').insert({ id: 1, hero_photo_id: 1 });
+
+    await migration.up(knex);
+
+    expect((await knex('events').where({ id: 1 }).first()).hero_photo_id).toBe(1);
+  });
+
+  it('makes a second insert of the same path impossible afterwards', async () => {
+    // The whole point. Without this the route is still racing, and the
+    // migration is recorded as applied.
+    await knex('photos').insert({ event_id: 1, external_relpath: 'a/x.jpg', source_origin: 'external' });
+
+    await migration.up(knex);
+
+    await expect(
+      knex('photos').insert({ event_id: 1, external_relpath: 'a/x.jpg', source_origin: 'external' })
+    ).rejects.toThrow(/unique/i);
+  });
+
+  it('still admits managed rows once the index exists', async () => {
+    await migration.up(knex);
+
+    await knex('photos').insert([
+      { event_id: 1, external_relpath: null, source_origin: 'managed' },
+      { event_id: 1, external_relpath: null, source_origin: 'managed' },
+    ]);
+
+    expect(await knex('photos').count('* as c').first()).toEqual({ c: 2 });
+  });
+
+  it('leaves nothing dangling behind the deleted row', async () => {
+    // SQLite never enforces the ON DELETE CASCADE these tables declare, so a
+    // bare delete strands biometric embeddings, feedback and marks pointing at
+    // a photo id that no longer exists — on every SQLite install.
+    await seedPair();
+    await knex('photo_faces').insert({ photo_id: 2, event_id: 1 });
+    await knex('image_access_logs').insert({ photo_id: 2 });
+
+    await migration.up(knex);
+
+    expect(await knex('photo_faces').where('photo_id', 2).first()).toBeUndefined();
+    expect(await knex('image_access_logs').where('photo_id', 2).first()).toBeUndefined();
+  });
+
+  it('does not carry the duplicate\'s faces over to the survivor', async () => {
+    // Both rows were scanned independently, so the survivor already holds its
+    // own embeddings. Moving these would fabricate a second copy of every face
+    // and split the person clusters built from them.
+    await seedPair();
+    await knex('photo_faces').insert([{ photo_id: 1, event_id: 1 }, { photo_id: 2, event_id: 1 }]);
+
+    await migration.up(knex);
+
+    expect(await knex('photo_faces').count('* as c').first()).toEqual({ c: 1 });
+  });
+
+  it('moves a guest comment to the survivor rather than deleting it', async () => {
+    // The duplicates were separate tiles in the grid, so a guest could have
+    // commented on either. Silently dropping that inside a fix for silent data
+    // loss would be its own bug.
+    await seedPair();
+    await knex('photo_feedback').insert({
+      photo_id: 2, event_id: 1, feedback_type: 'comment',
+      comment_text: 'lovely shot', guest_identifier: 'guest-a',
+    });
+
+    await migration.up(knex);
+
+    const rows = await knex('photo_feedback');
+    expect(rows).toHaveLength(1);
+    expect(rows[0].photo_id).toBe(1);
+    expect(rows[0].comment_text).toBe('lovely shot');
+  });
+
+  it('keeps both comments when the same guest commented on both tiles', async () => {
+    await seedPair();
+    await knex('photo_feedback').insert([
+      { photo_id: 1, event_id: 1, feedback_type: 'comment', comment_text: 'one', guest_identifier: 'g' },
+      { photo_id: 2, event_id: 1, feedback_type: 'comment', comment_text: 'two', guest_identifier: 'g' },
+    ]);
+
+    await migration.up(knex);
+
+    const rows = await knex('photo_feedback').orderBy('id');
+    expect(rows.map((r) => r.comment_text)).toEqual(['one', 'two']);
+    expect(rows.every((r) => r.photo_id === 1)).toBe(true);
+  });
+
+  it('does not double-count a like the same guest left on both tiles', async () => {
+    // Unlike comments, a like is a per-guest toggle: moving it would show two
+    // likes from one person.
+    await seedPair();
+    await knex('photo_feedback').insert([
+      { photo_id: 1, event_id: 1, feedback_type: 'like', guest_identifier: 'g' },
+      { photo_id: 2, event_id: 1, feedback_type: 'like', guest_identifier: 'g' },
+    ]);
+
+    await migration.up(knex);
+
+    expect(await knex('photo_feedback').count('* as c').first()).toEqual({ c: 1 });
+  });
+
+  it('moves a like from a guest the survivor has never seen', async () => {
+    await seedPair();
+    await knex('photo_feedback').insert({
+      photo_id: 2, event_id: 1, feedback_type: 'like', guest_identifier: 'other',
+    });
+
+    await migration.up(knex);
+
+    const rows = await knex('photo_feedback');
+    expect(rows).toHaveLength(1);
+    expect(rows[0].photo_id).toBe(1);
+  });
+
+  it('moves an admin mark, and drops it when that admin already marked the survivor', async () => {
+    // photo_admin_marks is UNIQUE(photo_id, admin_id), so a blind move would
+    // throw and abort the migration.
+    await seedPair();
+    await knex('photo_admin_marks').insert([
+      { photo_id: 1, event_id: 1, admin_id: 7, rating: 5 },
+      { photo_id: 2, event_id: 1, admin_id: 7, rating: 2 },
+      { photo_id: 2, event_id: 1, admin_id: 9, rating: 4 },
+    ]);
+
+    await migration.up(knex);
+
+    const rows = await knex('photo_admin_marks').orderBy('admin_id');
+    expect(rows.map((r) => [r.admin_id, r.rating])).toEqual([[7, 5], [9, 4]]);
+    expect(rows.every((r) => r.photo_id === 1)).toBe(true);
+  });
+
+  it('respects the transfer_files uniqueness when moving membership', async () => {
+    await seedPair();
+    await knex('transfer_files').insert([
+      { transfer_id: 3, photo_id: 1 },
+      { transfer_id: 3, photo_id: 2 },
+      { transfer_id: 4, photo_id: 2 },
+    ]);
+
+    await migration.up(knex);
+
+    const rows = await knex('transfer_files').orderBy('transfer_id');
+    expect(rows.map((r) => r.transfer_id)).toEqual([3, 4]);
+    expect(rows.every((r) => r.photo_id === 1)).toBe(true);
+  });
+
+  it('recomputes the survivor\'s feedback totals after reparenting rows', async () => {
+    // photos carries denormalized counters (migration 033). A survivor that
+    // now OWNS the feedback but still renders zero is the visible half of
+    // getting this wrong.
+    await seedPair();
+    await knex('photo_feedback').insert([
+      { photo_id: 2, event_id: 1, feedback_type: 'like', guest_identifier: 'g1' },
+      { photo_id: 2, event_id: 1, feedback_type: 'rating', rating: 4, guest_identifier: 'g1' },
+    ]);
+
+    await migration.up(knex);
+
+    const survivor = await knex('photos').where('id', 1).first();
+    expect(survivor.like_count).toBe(1);
+    expect(Number(survivor.average_rating)).toBe(4);
+    expect(survivor.feedback_count).toBe(1);
+  });
+
+  it('keeps two people who share a device apart', async () => {
+    // guest_identifier is per-device; guest_id is per-person (migration 078),
+    // and feedbackService scopes by guest_id when it is present. Keying on the
+    // identifier alone would read these as one person and delete a rating.
+    await seedPair();
+    await knex('photo_feedback').insert([
+      { photo_id: 1, event_id: 1, feedback_type: 'rating', rating: 5, guest_identifier: 'shared', guest_id: 10 },
+      { photo_id: 2, event_id: 1, feedback_type: 'rating', rating: 2, guest_identifier: 'shared', guest_id: 11 },
+    ]);
+
+    await migration.up(knex);
+
+    const rows = await knex('photo_feedback').orderBy('guest_id');
+    expect(rows.map((r) => [r.guest_id, r.rating])).toEqual([[10, 5], [11, 2]]);
+  });
+
+  it('still dedupes one person voting on both tiles', async () => {
+    await seedPair();
+    await knex('photo_feedback').insert([
+      { photo_id: 1, event_id: 1, feedback_type: 'like', guest_identifier: 'shared', guest_id: 10 },
+      { photo_id: 2, event_id: 1, feedback_type: 'like', guest_identifier: 'shared', guest_id: 10 },
+    ]);
+
+    await migration.up(knex);
+
+    expect(await knex('photo_feedback').count('* as c').first()).toEqual({ c: 1 });
+  });
+
+  it('rebuilds the people that lose members, rather than deleting faces raw', async () => {
+    // purgePhotoFaces is "called from every photo-deletion path" precisely
+    // because event_people counts and centroids are derived from the rows
+    // being removed. A bare delete leaves a ghost person behind.
+    await seedPair();
+    await knex('event_people').insert({ id: 5, event_id: 1, face_count: 1 });
+    await knex('photo_faces').insert({ photo_id: 2, event_id: 1, person_id: 5 });
+
+    await migration.up(knex);
+
+    expect(await knex('photo_faces').count('* as c').first()).toEqual({ c: 0 });
+    // The person had exactly one member and loses it, so it goes with it.
+    expect(await knex('event_people').where('id', 5).first()).toBeUndefined();
+  });
+
+  it('keeps a hidden moderation record from swallowing the visible replacement', async () => {
+    // feedbackService lets both coexist and counts only the visible one.
+    await seedPair();
+    await knex('photo_feedback').insert([
+      { photo_id: 1, event_id: 1, feedback_type: 'like', guest_identifier: 'g', is_hidden: true },
+      { photo_id: 2, event_id: 1, feedback_type: 'like', guest_identifier: 'g', is_hidden: false },
+    ]);
+
+    await migration.up(knex);
+
+    expect(await knex('photo_feedback').count('* as c').first()).toEqual({ c: 2 });
+  });
+
+  it('merges the independent halves of one admin\'s mark', async () => {
+    // rating and color_label are written independently, so the same admin can
+    // have rated one tile and coloured the other.
+    await seedPair();
+    await knex('photo_admin_marks').insert([
+      { photo_id: 1, event_id: 1, admin_id: 7, rating: 5, color_label: null },
+      { photo_id: 2, event_id: 1, admin_id: 7, rating: null, color_label: 'red' },
+    ]);
+
+    await migration.up(knex);
+
+    const rows = await knex('photo_admin_marks');
+    expect(rows).toHaveLength(1);
+    expect([rows[0].rating, rows[0].color_label]).toEqual([5, 'red']);
+  });
+
+  it('requeues the survivor when the duplicate held the only scan', async () => {
+    // Otherwise the sole embeddings go with the purge and nothing re-queues:
+    // the photo just silently stops having a face.
+    await seedPair();
+    await knex('photo_faces').insert({ photo_id: 2, event_id: 1 });
+
+    await migration.up(knex);
+
+    expect((await knex('photos').where('id', 1).first()).face_status).toBe('pending');
+  });
+
+  it('carries the duplicate\'s views and downloads over', async () => {
+    await seedPair();
+    await knex('photos').where('id', 1).update({ view_count: 2, download_count: 1 });
+    await knex('photos').where('id', 2).update({ view_count: 5, download_count: 3 });
+
+    await migration.up(knex);
+
+    const survivor = await knex('photos').where('id', 1).first();
+    expect([survivor.view_count, survivor.download_count]).toEqual([7, 4]);
+  });
+
+  it('fails loudly rather than recording itself applied without the index', async () => {
+    // Swallowing a failed CREATE INDEX would leave the install permanently
+    // racy — the in-flight guard only covers one process — with nothing to
+    // trigger a retry. Driven through the helper the migration calls, against
+    // a table that still holds duplicates — i.e. what it would face if the
+    // dedupe above had not achieved uniqueness.
+    await seedPair();
+    const { createExternalRelpathIndex } = require('../../src/services/externalPhotoDedupe');
+
+    await expect(createExternalRelpathIndex(knex)).rejects.toThrow(/unique/i);
+  });
+
+  it('invalidates the pre-built download zip for the affected event', async () => {
+    // The cached archive still contains the rows just removed, and every
+    // ordinary photo-deletion path invalidates it for exactly that reason.
+    // getZipInfo treats a cleared record as a miss and rebuilds on request.
+    await seedPair();
+    await knex('events').insert({
+      id: 1, download_zip_path: 'events/active/x/.download-cache/all.zip',
+      download_zip_generated_at: '2026-01-01',
+    });
+
+    await migration.up(knex);
+
+    const ev = await knex('events').where('id', 1).first();
+    expect(ev.download_zip_path).toBeNull();
+    expect(ev.download_zip_generated_at).toBeNull();
+  });
+
+  it('leaves an untouched event\'s zip alone', async () => {
+    await seedPair();
+    await knex('events').insert([
+      { id: 1, download_zip_path: 'a.zip', download_zip_generated_at: '2026-01-01' },
+      { id: 2, download_zip_path: 'b.zip', download_zip_generated_at: '2026-01-01' },
+    ]);
+
+    await migration.up(knex);
+
+    expect((await knex('events').where('id', 2).first()).download_zip_path).toBe('b.zip');
+  });
+
+  it('is idempotent', async () => {
+    await knex('photos').insert([
+      { event_id: 1, external_relpath: 'a/x.jpg', source_origin: 'external' },
+      { event_id: 1, external_relpath: 'a/x.jpg', source_origin: 'external' },
+    ]);
+
+    await migration.up(knex);
+    const once = await rows();
+    await migration.up(knex);
+
+    expect(await rows()).toEqual(once);
+  });
+
+  it('rolls back to an unconstrained table', async () => {
+    await migration.up(knex);
+    await migration.down(knex);
+
+    await knex('photos').insert([
+      { event_id: 1, external_relpath: 'a/x.jpg', source_origin: 'external' },
+      { event_id: 1, external_relpath: 'a/x.jpg', source_origin: 'external' },
+    ]);
+    expect(await knex('photos').count('* as c').first()).toEqual({ c: 2 });
+  });
+
+  it('no-ops before 041 has added the column', async () => {
+    await knex.schema.dropTableIfExists('photos');
+    await knex.schema.createTable('photos', (t) => { t.increments('id').primary(); });
+
+    await expect(migration.up(knex)).resolves.toBeUndefined();
+  });
+});
diff --git a/backend/__tests__/services/externalPhotoDedupe.restore.test.js b/backend/__tests__/services/externalPhotoDedupe.restore.test.js
new file mode 100644
index 00000000..2fca042d
--- /dev/null
+++ b/backend/__tests__/services/externalPhotoDedupe.restore.test.js
@@ -0,0 +1,92 @@
+/**
+ * A pre-#1162 backup must still restore (#1162 review).
+ *
+ * `replaceAllTables` suspends FOREIGN KEY enforcement for the load — Postgres
+ * via `session_replication_role = replica`, SQLite via `defer_foreign_keys` —
+ * but neither of those suspends a UNIQUE index. An archive taken before
+ * migration 186 carries exactly the duplicate photo rows that migration
+ * removes, so the batchInsert would hit the new index and roll the entire
+ * restore back, after every table had already been emptied.
+ *
+ * These pin the drop → load → dedupe → recreate sequence the restore now
+ * performs, and the failure it exists to prevent.
+ */
+
+const path = require('path');
+const fs = require('fs');
+const os = require('os');
+
+const {
+  dedupeExternalPhotos,
+  createExternalRelpathIndex,
+  dropExternalRelpathIndex,
+} = require('../../src/services/externalPhotoDedupe');
+
+describe('restoring an archive that predates the unique index (#1162)', () => {
+  let knex; let tmpDir;
+
+  // What a pre-186 archive's photos.ndjson holds for a racing import: the same
+  // file twice, sub-millisecond apart.
+  const ARCHIVE_ROWS = [
+    { id: 1, event_id: 1, external_relpath: 'Trip/a.jpg', source_origin: 'external' },
+    { id: 2, event_id: 1, external_relpath: 'Trip/a.jpg', source_origin: 'external' },
+    { id: 3, event_id: 1, external_relpath: 'Trip/b.jpg', source_origin: 'external' },
+  ];
+
+  beforeAll(async () => {
+    tmpDir = await fs.promises.mkdtemp(path.join(os.tmpdir(), 'picpeak-restore-dedupe-'));
+    knex = require('knex')({
+      client: 'sqlite3',
+      connection: { filename: path.join(tmpDir, 'db.sqlite') },
+      useNullAsDefault: true,
+    });
+  });
+
+  afterAll(async () => {
+    if (knex) await knex.destroy();
+    await fs.promises.rm(tmpDir, { recursive: true, force: true }).catch(() => {});
+  });
+
+  beforeEach(async () => {
+    await knex.schema.dropTableIfExists('photos');
+    await knex.schema.createTable('photos', (t) => {
+      t.integer('id').primary();
+      t.integer('event_id');
+      t.string('external_relpath');
+      t.string('thumbnail_path');
+      t.string('source_origin').defaultTo('managed');
+    });
+    await createExternalRelpathIndex(knex);
+  });
+
+  it('would abort the whole restore without the drop', async () => {
+    // The regression, stated directly: this is what the target instance does
+    // today when handed a legacy archive.
+    await expect(knex.batchInsert('photos', ARCHIVE_ROWS, 100)).rejects.toThrow(/unique/i);
+  });
+
+  it('loads, dedupes and comes back constrained', async () => {
+    await dropExternalRelpathIndex(knex);
+    await knex.batchInsert('photos', ARCHIVE_ROWS, 100);
+
+    const removed = await dedupeExternalPhotos(knex);
+    await createExternalRelpathIndex(knex);
+
+    expect(removed).toBe(1);
+    expect((await knex('photos').orderBy('id')).map((r) => r.external_relpath))
+      .toEqual(['Trip/a.jpg', 'Trip/b.jpg']);
+    // The target must not be left unprotected by the restore that dropped it.
+    await expect(
+      knex('photos').insert({ id: 9, event_id: 1, external_relpath: 'Trip/b.jpg', source_origin: 'external' })
+    ).rejects.toThrow(/unique/i);
+  });
+
+  it('is a no-op for an archive that has no duplicates', async () => {
+    await dropExternalRelpathIndex(knex);
+    await knex.batchInsert('photos', ARCHIVE_ROWS.slice(1), 100);
+
+    expect(await dedupeExternalPhotos(knex)).toBe(0);
+    await expect(createExternalRelpathIndex(knex)).resolves.toBeUndefined();
+    expect(await knex('photos').count('* as c').first()).toEqual({ c: 2 });
+  });
+});
diff --git a/backend/migrations/core/186_external_relpath_unique.js b/backend/migrations/core/186_external_relpath_unique.js
new file mode 100644
index 00000000..112c5f66
--- /dev/null
+++ b/backend/migrations/core/186_external_relpath_unique.js
@@ -0,0 +1,60 @@
+/**
+ * Migration 186: one row per external file per event (#1162).
+ *
+ * The import route checked for an existing external_relpath and then inserted,
+ * with an fs.stat and a sharp().metadata() call sitting in between — a window
+ * wide enough that two overlapping imports of the same folder each see "not
+ * there" and both insert. Nothing at the storage layer stopped them: 041
+ * created only a NON-unique (event_id, source_origin) index. A reporter's
+ * event ended up holding 8004 rows for 6012 distinct paths.
+ *
+ * So this does two things: clear the duplicates that already exist, and add
+ * the constraint that makes the race unwinnable from here on.
+ *
+ * The work — which row survives, what happens to the guest feedback and admin
+ * marks hanging off the loser, and why the dependent rows are deleted by hand
+ * rather than left to ON DELETE CASCADE — lives in
+ * services/externalPhotoDedupe.js, because a .picpeak restore has to run it
+ * too: the archive carries the photos table verbatim, so a pre-#1162 backup
+ * would otherwise hit the unique index mid-restore and roll the whole thing
+ * back.
+ *
+ * Irreversible by design: down() drops the index but cannot resurrect the
+ * deleted rows. They were never distinct data — the same file counted twice.
+ *
+ * What it does NOT do is delete the duplicates' thumbnail files. Those are
+ * `ext<id>_<name>` keys under the thumbnail root, and a migration is the wrong
+ * place to reach into storage — the backend may be pointed at S3, and a failed
+ * object delete must not fail the schema change. They are left behind as
+ * unreferenced bytes; the storage figures on the dashboard count them, which
+ * is the correct answer to "what is on the disk".
+ */
+
+const {
+  dedupeExternalPhotos,
+  createExternalRelpathIndex,
+  dropExternalRelpathIndex,
+} = require('../../src/services/externalPhotoDedupe');
+
+exports.up = async function(knex) {
+  if (!(await knex.schema.hasTable('photos'))) return;
+  if (!(await knex.schema.hasColumn('photos', 'external_relpath'))) return;
+
+  const removed = await dedupeExternalPhotos(knex);
+  if (removed) {
+    console.log(`186_external_relpath_unique: removed ${removed} duplicate external photo row(s)`);
+  }
+
+  // Deliberately unguarded. Recording this migration as applied without the
+  // index would leave the install permanently racy — the in-flight set only
+  // covers one process, and the route's unique-violation path cannot converge
+  // without a constraint to violate — with nothing to trigger a retry. A
+  // failure here means the dedupe above did not achieve uniqueness, which is
+  // worth stopping the upgrade for.
+  await createExternalRelpathIndex(knex);
+};
+
+exports.down = async function(knex) {
+  if (!(await knex.schema.hasTable('photos'))) return;
+  await dropExternalRelpathIndex(knex);
+};
diff --git a/backend/src/routes/adminExternalMedia.js b/backend/src/routes/adminExternalMedia.js
index d4507ed6..98a60be2 100644
--- a/backend/src/routes/adminExternalMedia.js
+++ b/backend/src/routes/adminExternalMedia.js
@@ -9,9 +9,24 @@ const { db, logActivity } = require('../database/db');
 const sharp = require('sharp');
 const logger = require('../utils/logger');
 const { generateThumbnail } = require('../services/imageProcessor');
+const { isUniqueViolation } = require('../utils/dbErrors');
 
 const router = express.Router();
 
+// Events with an import running in THIS process (#1162).
+//
+// The second line of defence, not the first: migration 186 puts a unique index
+// on (event_id, external_relpath), and that is what actually makes a duplicate
+// impossible — it holds across replicas, across restarts, and against anything
+// that inserts external rows without going through this route.
+//
+// This set exists for the reason the duplicates got filed in the first place:
+// a large tree takes long enough that the run LOOKS hung, so admins click
+// again. Letting that second run walk the whole tree only to have every insert
+// bounce off the index wastes minutes of CPU and reports a nonsense
+// `skipped: 6012` back. Failing it immediately with 409 says what happened.
+const importsInFlight = new Set();
+
 // GET /api/admin/external-media/list?path=relative/dir
 router.get('/list', adminAuth, requirePermission('photos.view'), async (req, res) => {
   try {
@@ -50,8 +65,14 @@ async function walkDir(dir, baseDir) {
 // POST /api/admin/events/:id/import-external
 // Body: { external_path: string, recursive?: boolean, map?: { individual?: string, collages?: string } }
 router.post('/events/:id/import-external', adminAuth, requirePermission('photos.upload'), requireEventOwnership, async (req, res) => {
+  const eventId = parseInt(req.params.id);
+  if (importsInFlight.has(eventId)) {
+    return res.status(409).json({
+      error: 'An import is already running for this event. Wait for it to finish before starting another.'
+    });
+  }
+  importsInFlight.add(eventId);
   try {
-    const eventId = parseInt(req.params.id);
     const { external_path, recursive = true, map = { individual: 'individual', collages: 'collages' } } = req.body || {};
     if (!external_path) return res.status(400).json({ error: 'external_path is required' });
 
@@ -155,7 +176,13 @@ router.post('/events/:id/import-external', adminAuth, requirePermission('photos.
       if (segs[0] === map.individual) type = 'individual';
 
       try {
-        // Check if already exists (by external_relpath)
+        // Fast path only. This SELECT settles the common case — a re-import of
+        // a folder already in the event — without paying for a stat and a
+        // Sharp metadata read per file. It is NOT the guard: those two calls
+        // sit between here and the INSERT below, which is exactly the window
+        // two overlapping imports both walked through (#1162). The unique
+        // index from migration 186 is the guard, and the catch below is how
+        // this loop converges when it fires.
         const exists = await db('photos')
           .where({ event_id: eventId, external_relpath: f.rel })
           .first();
@@ -173,21 +200,33 @@ router.post('/events/:id/import-external', adminAuth, requirePermission('photos.
           logger.warn(`Could not extract dimensions for ${f.rel}: ${dimErr.message}`);
         }
 
-        const inserted = await db('photos')
-          .insert({
-            event_id: eventId,
-            filename: f.name,
-            // Keep path as a hint for legacy code but not used for resolution in external mode
-            path: path.join(event.slug, f.name),
-            thumbnail_path: null,
-            type,
-            size_bytes: stats.size,
-            width,
-            height,
-            source_origin: 'external',
-            external_relpath: f.rel
-          })
-          .returning('id');
+        let inserted;
+        try {
+          inserted = await db('photos')
+            .insert({
+              event_id: eventId,
+              filename: f.name,
+              // Keep path as a hint for legacy code but not used for resolution in external mode
+              path: path.join(event.slug, f.name),
+              thumbnail_path: null,
+              type,
+              size_bytes: stats.size,
+              width,
+              height,
+              source_origin: 'external',
+              external_relpath: f.rel
+            })
+            .returning('id');
+        } catch (insertErr) {
+          // Another writer inserted this exact path while we were reading
+          // metadata. That is the outcome the index exists to produce, and it
+          // is a skip rather than a failure — the row is there, it just isn't
+          // ours. Counting it as `skipped` keeps the reported totals honest;
+          // before the index this landed in the outer catch as a nameless
+          // failure, or (more often) never fired at all and duplicated the row.
+          if (isUniqueViolation(insertErr)) { skipped++; continue; }
+          throw insertErr;
+        }
 
         const photoId = Array.isArray(inserted) && inserted.length
           ? (typeof inserted[0] === 'object' ? inserted[0].id : inserted[0])
@@ -275,6 +314,11 @@ router.post('/events/:id/import-external', adminAuth, requirePermission('photos.
       error: error.message
     });
     res.status(500).json({ error: 'Failed to import external media' });
+  } finally {
+    // In `finally` and not at the end of `try`: an import that throws must
+    // still release the event, or a single failure locks out every retry
+    // until the process restarts.
+    importsInFlight.delete(eventId);
   }
 });
 
diff --git a/backend/src/services/externalPhotoDedupe.js b/backend/src/services/externalPhotoDedupe.js
new file mode 100644
index 00000000..8d1a8e51
--- /dev/null
+++ b/backend/src/services/externalPhotoDedupe.js
@@ -0,0 +1,389 @@
+/**
+ * One row per external file per event (#1162).
+ *
+ * The import route used to check for an existing external_relpath and then
+ * insert, with an fs.stat and a Sharp decode in between — wide enough that two
+ * overlapping imports both walked through it. A reporter's event held 8004 rows
+ * for 6012 distinct paths.
+ *
+ * This lives in a service rather than inside migration 186 because it has two
+ * callers. The migration is one. The other is a .picpeak restore: the archive
+ * carries the photos table verbatim, so a backup taken before this fix lands
+ * duplicate rows into a schema that now has a unique index on them — and
+ * neither Postgres' `session_replication_role = replica` nor SQLite's
+ * `defer_foreign_keys` disables a UNIQUE index, so batchInsert would abort the
+ * whole restore after every table had already been emptied.
+ *
+ * DELETING dependent rows explicitly, rather than trusting ON DELETE CASCADE,
+ * is the load-bearing part. Every FK into photos declares CASCADE, but PicPeak
+ * does not set `PRAGMA foreign_keys = ON` — the codebase says so in as many
+ * words where it deletes an event (adminEvents/helpers.js:245-249) — so on
+ * every SQLite install the cascade is inert and a bare delete would leave
+ * dangling face embeddings, feedback and marks behind. The same reason
+ * `hero_photo_id` is repointed by hand: its SET NULL is inert there too, so
+ * without it a SQLite install keeps a hero pointing at a row that is gone.
+ *
+ * Guest and admin state is MOVED to the survivor where it can be, not
+ * discarded. The duplicates were separate tiles in the grid, so a guest's
+ * comment or an admin's rating could legitimately be attached to either, and
+ * silently deleting it inside a fix for silent data loss would be its own bug.
+ * Where the target already holds an equivalent row — the same guest's like on
+ * the same photo, the same admin's mark, the same transfer's entry — the loser
+ * is dropped instead, because those tables mean "one per (photo, actor)" and
+ * moving would either violate a unique constraint or double-count.
+ *
+ * photo_faces is the deliberate exception: both rows were scanned
+ * independently, so the survivor already has its own embeddings and moving the
+ * duplicate's would fabricate a second copy of every face and split the
+ * person clusters built from them.
+ */
+
+const { isUniqueViolation } = require('../utils/dbErrors');
+
+const CHUNK = 400; // SQLite caps a statement at 999 bound parameters.
+// Joins the parts of an equivalence key. Escaped, not a literal: a raw NUL in
+// the source makes git classify this whole file as binary and hide its diffs.
+const KEY_SEP = '\u0000';
+const INDEX_NAME = 'photos_event_external_relpath_uniq';
+
+const chunked = (arr) => {
+  const out = [];
+  for (let i = 0; i < arr.length; i += CHUNK) out.push(arr.slice(i, i + CHUNK));
+  return out;
+};
+
+/** Pure log rows — nothing is lost by dropping them with the duplicate. */
+const LOG_TABLES = [
+  ['image_access_logs', 'photo_id'],
+  ['transfer_downloads', 'photo_id'],
+];
+
+/**
+ * Tables holding one row per (photo, actor). `keys` is what makes two rows
+ * equivalent, so a move that would collide becomes a delete instead.
+ */
+const MOVE_TABLES = [
+  {
+    table: 'photo_feedback',
+    // Guest identity, the way feedbackService defines it: guest_id when the
+    // gallery uses per-person guests (migration 078), guest_identifier
+    // otherwise — the same COALESCE its own duplicate-check and stats
+    // aggregate use (feedbackService.js:208, :559). Keying on
+    // guest_identifier alone would treat two DIFFERENT people sharing a
+    // device as one and delete one of their ratings.
+    identity: (row) => (row.guest_id != null ? `id:${row.guest_id}` : `anon:${row.guest_identifier}`),
+    // is_hidden is part of the identity, not noise: feedbackService lets a
+    // moderator-hidden row coexist with the guest's visible replacement and
+    // excludes hidden rows from the counts. Without it the visible row is
+    // dropped as redundant against the hidden one.
+    keys: ['feedback_type', 'is_hidden'],
+    // A comment is distinct content, never a per-guest toggle: two comments
+    // from one guest are two comments, so they always move.
+    alwaysMove: (row) => row.feedback_type === 'comment',
+  },
+  {
+    table: 'photo_admin_marks',
+    keys: ['admin_id'],
+    // rating and color_label are independently writable, so the same admin can
+    // have rated one tile and colour-labelled the other. Dropping the loser
+    // outright would lose a half the survivor's row has no value for.
+    mergeFields: ['rating', 'color_label'],
+  },
+  { table: 'transfer_files', keys: ['transfer_id'] },
+];
+
+const equivalenceKey = (spec, row) => [
+  spec.identity ? spec.identity(row) : '',
+  ...spec.keys.map((k) => row[k]),
+].join(KEY_SEP);
+
+async function repointColumn(knex, table, column, doomedToSurvivor) {
+  if (!(await knex.schema.hasTable(table))) return;
+  if (!(await knex.schema.hasColumn(table, column))) return;
+  for (const [doomed, survivor] of doomedToSurvivor) {
+    await knex(table).where(column, doomed).update({ [column]: survivor });
+  }
+}
+
+async function moveOrDrop(knex, spec, doomedToSurvivor, touched) {
+  if (!(await knex.schema.hasTable(spec.table))) return;
+
+  for (const [doomed, survivor] of doomedToSurvivor) {
+    const rows = await knex(spec.table).where('photo_id', doomed);
+    if (!rows.length) continue;
+    if (touched) touched.add(survivor);
+
+    const existing = await knex(spec.table).where('photo_id', survivor);
+    const taken = new Set(existing.map((r) => equivalenceKey(spec, r)));
+
+    for (const row of rows) {
+      const key = equivalenceKey(spec, row);
+      const move = (spec.alwaysMove && spec.alwaysMove(row)) || !taken.has(key);
+      if (!move) {
+        // Before dropping the loser, hand over any field the winner has no
+        // value for — otherwise an independently-set half goes with it.
+        if (spec.mergeFields) {
+          const winner = existing.find((r) => equivalenceKey(spec, r) === key);
+          const fill = {};
+          for (const field of spec.mergeFields) {
+            if (winner && winner[field] == null && row[field] != null) fill[field] = row[field];
+          }
+          if (winner && Object.keys(fill).length) {
+            await knex(spec.table).where('id', winner.id).update(fill);
+            Object.assign(winner, fill);
+          }
+        }
+        await knex(spec.table).where('id', row.id).del();
+        continue;
+      }
+      try {
+        await knex(spec.table).where('id', row.id).update({ photo_id: survivor });
+        taken.add(key);
+      } catch (err) {
+        // A unique constraint we did not model. The row is redundant with one
+        // the survivor already has, so dropping it is correct — but anything
+        // else must surface rather than leave a dangling photo_id behind.
+        if (!isUniqueViolation(err)) throw err;
+        await knex(spec.table).where('id', row.id).del();
+      }
+    }
+  }
+}
+
+/**
+ * Remove every photo row in `doomedToSurvivor`, moving or dropping the state
+ * that hangs off it first. Safe on both engines and on schemas that predate
+ * any of the dependent tables.
+ */
+async function deleteDuplicatePhotos(knex, doomedToSurvivor) {
+  const doomed = [...doomedToSurvivor.keys()];
+  if (!doomed.length) return 0;
+
+  // SET NULL is as inert as CASCADE on SQLite, so an event whose hero happened
+  // to be the duplicate would silently lose its hero image.
+  await repointColumn(knex, 'events', 'hero_photo_id', doomedToSurvivor);
+  await repointColumn(knex, 'photo_categories', 'hero_photo_id', doomedToSurvivor);
+
+  const feedbackTouched = new Set();
+  for (const spec of MOVE_TABLES) {
+    await moveOrDrop(knex, spec, doomedToSurvivor, spec.table === 'photo_feedback' ? feedbackTouched : null);
+  }
+
+  // photos carries denormalized feedback totals (migration 033:
+  // feedback_count, like_count, average_rating, favorite_count, and later
+  // reaction/colour counts). Reparenting rows without recomputing leaves a
+  // survivor that now OWNS feedback still rendering zero.
+  if (feedbackTouched.size && await knex.schema.hasColumn('photos', 'feedback_count')) {
+    const feedbackService = require('./feedbackService');
+    for (const survivor of feedbackTouched) {
+      await feedbackService.updatePhotoFeedbackStats(survivor, knex);
+    }
+  }
+
+  for (const [table, column] of LOG_TABLES) {
+    if (!(await knex.schema.hasTable(table))) continue;
+    for (const ids of chunked(doomed)) await knex(table).whereIn(column, ids).del();
+  }
+
+  // Both rows were scanned, so the survivor has its own faces; moving the
+  // duplicate's would double every embedding and split the person clusters.
+  //
+  // Through purgePhotoFaces, not a raw delete: deleting the rows is only half
+  // of it. event_people counts and centroids are derived from the faces being
+  // removed, and #1132's separation snapshots hold a COPY of each side's
+  // centroid — so a bare delete leaves ghost or inflated people and vectors
+  // built from photos that no longer exist. faceProcessor says as much: it is
+  // "called from every photo-deletion path".
+  if (await knex.schema.hasTable('photo_faces')) {
+    // If the ONLY completed scan of this file belonged to the duplicate, the
+    // purge below takes the sole embeddings with it and nothing re-queues the
+    // survivor — it just silently stops having a face. Mark those for a
+    // rescan; the worker picks up 'pending' on its own.
+    const needsRescan = [];
+    for (const [doomedId, survivorId] of doomedToSurvivor) {
+      if (!(await knex('photo_faces').where('photo_id', doomedId).first())) continue;
+      if (!(await knex('photo_faces').where('photo_id', survivorId).first())) needsRescan.push(survivorId);
+    }
+
+    let purgePhotoFaces = null;
+    try {
+      ({ purgePhotoFaces } = require('./faceProcessor'));
+    } catch (err) {
+      // Face detection is optional; an install without it still needs the rows
+      // gone so nothing dangles on SQLite.
+      purgePhotoFaces = null;
+    }
+    if (purgePhotoFaces) {
+      for (const id of doomed) await purgePhotoFaces(id, knex);
+    } else {
+      for (const ids of chunked(doomed)) await knex('photo_faces').whereIn('photo_id', ids).del();
+    }
+
+    if (needsRescan.length && await knex.schema.hasColumn('photos', 'face_status')) {
+      for (const ids of chunked(needsRescan)) {
+        await knex('photos').whereIn('id', ids).update({ face_status: 'pending' });
+      }
+    }
+  }
+
+  // Real interactions, recorded per row. Deleting the duplicate would quietly
+  // lower the engagement the admin grid shows for a photo people did view and
+  // download.
+  if (await knex.schema.hasColumn('photos', 'view_count')) {
+    for (const [doomedId, survivorId] of doomedToSurvivor) {
+      const from = await knex('photos').where('id', doomedId)
+        .select('view_count', 'download_count').first();
+      if (!from) continue;
+      const add = {};
+      if (from.view_count) add.view_count = knex.raw('COALESCE(view_count, 0) + ?', [from.view_count]);
+      if (from.download_count) add.download_count = knex.raw('COALESCE(download_count, 0) + ?', [from.download_count]);
+      if (Object.keys(add).length) await knex('photos').where('id', survivorId).update(add);
+    }
+  }
+
+  for (const ids of chunked(doomed)) await knex('photos').whereIn('id', ids).del();
+
+  // The pre-built "download everything" zip still contains the rows just
+  // removed. Every ordinary photo-deletion path calls
+  // downloadZipService.invalidate for this reason (adminPhotos.js:450 and
+  // friends) — but that service carries debounce timers and a regeneration
+  // queue, which is not something a migration should be starting. Clearing the
+  // columns is the durable half of what invalidate does: getZipInfo already
+  // treats a missing or absent record as a cache miss and rebuilds on the next
+  // request, so guests stop receiving an archive containing deleted duplicates.
+  //
+  // The stale object itself is left for the same reason the duplicates'
+  // thumbnails are — a migration is the wrong place to reach into storage,
+  // which may be S3.
+  if (await knex.schema.hasColumn('events', 'download_zip_path')) {
+    const affected = [...new Set([...doomedToSurvivor.values()])];
+    const eventIds = affected.length
+      ? (await knex('photos').whereIn('id', affected).distinct('event_id')).map((r) => r.event_id)
+      : [];
+    for (const ids of chunked(eventIds)) {
+      await knex('events').whereIn('id', ids).update({
+        download_zip_path: null,
+        download_zip_generated_at: null,
+      });
+    }
+  }
+
+  return doomed.length;
+}
+
+/**
+ * Which rows are duplicates, and which one survives.
+ *
+ * Survivor: the lowest id that has a thumbnail_path, else the lowest id.
+ * Thumbnails are generated per row during import, so on a duplicated pair both
+ * usually have one and the tie-break never fires — but an import killed
+ * mid-flight leaves rows without, and dropping the one that HAS the thumbnail
+ * would blank a grid tile for no reason.
+ */
+async function planDedupe(knex) {
+  const dupKeys = await knex('photos')
+    .whereNotNull('external_relpath')
+    .select('event_id')
+    .count('* as c')
+    .groupBy('event_id', 'external_relpath')
+    .havingRaw('count(*) > 1');
+
+  const doomedToSurvivor = new Map();
+
+  for (const eventId of new Set(dupKeys.map((r) => r.event_id))) {
+    const rows = await knex('photos')
+      .where('event_id', eventId)
+      .whereNotNull('external_relpath')
+      .select('id', 'external_relpath', 'thumbnail_path')
+      .orderBy('id', 'asc');
+
+    const byPath = new Map();
+    for (const row of rows) {
+      const group = byPath.get(row.external_relpath);
+      if (group) group.push(row);
+      else byPath.set(row.external_relpath, [row]);
+    }
+
+    for (const group of byPath.values()) {
+      if (group.length < 2) continue;
+      const survivor = group.find((r) => r.thumbnail_path) || group[0];
+      for (const row of group) {
+        if (row.id !== survivor.id) doomedToSurvivor.set(row.id, survivor.id);
+      }
+    }
+  }
+
+  return doomedToSurvivor;
+}
+
+/** @returns {Promise<number>} how many duplicate rows were removed. */
+async function dedupeExternalPhotos(knex) {
+  if (!(await knex.schema.hasTable('photos'))) return 0;
+  if (!(await knex.schema.hasColumn('photos', 'external_relpath'))) return 0;
+  return deleteDuplicatePhotos(knex, await planDedupe(knex));
+}
+
+/** Is the index actually there? Asked of the catalog, not inferred. */
+async function externalRelpathIndexExists(knex) {
+  const isPg = knex.client && knex.client.config && knex.client.config.client === 'pg';
+  const row = isPg
+    ? await knex('pg_indexes').where('indexname', INDEX_NAME).first()
+    : await knex('sqlite_master').where({ type: 'index', name: INDEX_NAME }).first();
+  return !!row;
+}
+
+/**
+ * The error a failed index MUST raise.
+ *
+ * Deliberately carries no `code`. run-migrations-safe.js treats 23505, 42P07,
+ * 42701 and 42710 as "schema already exists" and marks the migration applied
+ * (run-migrations-safe.js:138) — and a CREATE UNIQUE INDEX that finds
+ * duplicate rows raises exactly 23505 on Postgres. Letting the driver's error
+ * through would therefore record 186 as done on an install that never got the
+ * index, with nothing to trigger a retry: the precise outcome the throw
+ * exists to prevent.
+ */
+function indexFailure(detail) {
+  return new Error(
+    `Could not create ${INDEX_NAME}: ${detail}. The photos table still holds `
+    + 'duplicate (event_id, external_relpath) rows — most likely inserted by a '
+    + 'concurrent import while this migration ran. Stop other writers and re-run.'
+  );
+}
+
+/**
+ * Partial, so the managed rows — which all carry NULL — are not indexed at
+ * all. Both engines treat NULLs as distinct in a unique index, so a plain one
+ * would also be correct, but it would carry every managed photo for no query
+ * that ever uses it.
+ */
+async function createExternalRelpathIndex(knex) {
+  try {
+    await knex.raw(
+      `CREATE UNIQUE INDEX IF NOT EXISTS ${INDEX_NAME} `
+      + 'ON photos (event_id, external_relpath) WHERE external_relpath IS NOT NULL'
+    );
+  } catch (err) {
+    throw indexFailure(err.message);
+  }
+  // IF NOT EXISTS makes the statement itself a poor witness, and a replica
+  // inserting a duplicate between the dedupe and this lock is a real rolling-
+  // deploy shape. Ask the catalog.
+  if (!(await externalRelpathIndexExists(knex))) {
+    throw indexFailure('the index is absent afterwards');
+  }
+}
+
+async function dropExternalRelpathIndex(knex) {
+  await knex.raw(`DROP INDEX IF EXISTS ${INDEX_NAME}`);
+}
+
+module.exports = {
+  dedupeExternalPhotos,
+  externalRelpathIndexExists,
+  deleteDuplicatePhotos,
+  planDedupe,
+  createExternalRelpathIndex,
+  dropExternalRelpathIndex,
+  INDEX_NAME,
+};
diff --git a/backend/src/services/feedbackService.js b/backend/src/services/feedbackService.js
index e983cf37..6a28be1e 100644
--- a/backend/src/services/feedbackService.js
+++ b/backend/src/services/feedbackService.js
@@ -541,27 +541,32 @@ class FeedbackService {
   }
 
   /**
-   * Update photo feedback statistics
+   * Update photo feedback statistics.
+   *
+   * `trx` so a caller running outside the request path — the duplicate-photo
+   * dedupe (#1162), which reparents feedback rows and must leave the
+   * survivor's denormalized totals correct — can recompute on its own
+   * connection.
    */
-  async updatePhotoFeedbackStats(photoId) {
+  async updatePhotoFeedbackStats(photoId, trx = db) {
     try {
       // Get aggregated stats
-      const stats = await db('photo_feedback')
+      const stats = await trx('photo_feedback')
         .where('photo_id', photoId)
         .where('is_hidden', false)
         .select(
-          db.raw('COUNT(CASE WHEN feedback_type = ? AND is_approved = ? THEN 1 END) as comment_count', ['comment', formatBoolean(true)]),
-          db.raw('COUNT(CASE WHEN feedback_type = ? THEN 1 END) as like_count', ['like']),
-          db.raw('COUNT(CASE WHEN feedback_type = ? THEN 1 END) as favorite_count', ['favorite']),
-          db.raw('COUNT(CASE WHEN feedback_type = ? THEN 1 END) as reaction_count', ['reaction']),
-          db.raw('COUNT(CASE WHEN feedback_type = ? THEN 1 END) as color_label_count', ['color_label']),
-          db.raw('AVG(CASE WHEN feedback_type = ? THEN rating END) as average_rating', ['rating']),
-          db.raw('COUNT(DISTINCT COALESCE(CAST(guest_id AS VARCHAR), guest_identifier)) as feedback_count')
+          trx.raw('COUNT(CASE WHEN feedback_type = ? AND is_approved = ? THEN 1 END) as comment_count', ['comment', formatBoolean(true)]),
+          trx.raw('COUNT(CASE WHEN feedback_type = ? THEN 1 END) as like_count', ['like']),
+          trx.raw('COUNT(CASE WHEN feedback_type = ? THEN 1 END) as favorite_count', ['favorite']),
+          trx.raw('COUNT(CASE WHEN feedback_type = ? THEN 1 END) as reaction_count', ['reaction']),
+          trx.raw('COUNT(CASE WHEN feedback_type = ? THEN 1 END) as color_label_count', ['color_label']),
+          trx.raw('AVG(CASE WHEN feedback_type = ? THEN rating END) as average_rating', ['rating']),
+          trx.raw('COUNT(DISTINCT COALESCE(CAST(guest_id AS VARCHAR), guest_identifier)) as feedback_count')
         )
         .first();
 
       // Update photo table
-      await db('photos')
+      await trx('photos')
         .where('id', photoId)
         .update({
           feedback_count: stats.feedback_count || 0,
diff --git a/backend/src/services/picpeakImportService.js b/backend/src/services/picpeakImportService.js
index fa3beab7..32f27c97 100644
--- a/backend/src/services/picpeakImportService.js
+++ b/backend/src/services/picpeakImportService.js
@@ -27,6 +27,11 @@ const { hasColumnCached } = require('../utils/schemaCache');
 const { setSessionsValidAfter } = require('../utils/sessionCutoff');
 const logger = require('../utils/logger');
 const { PICPEAK_FORMAT_VERSION, EXCLUDED_TABLES, listDataTables } = require('./picpeakExportService');
+const {
+  dedupeExternalPhotos,
+  createExternalRelpathIndex,
+  dropExternalRelpathIndex,
+} = require('./externalPhotoDedupe');
 
 const isPostgres = () => knexConfig.client === 'pg';
 
@@ -348,6 +353,19 @@ async function replaceAllTables(tables, dataDir, currentAdmin, roleSnapshot, { c
       await trx.raw('PRAGMA defer_foreign_keys = ON');
     }
 
+    // Suspending FK enforcement does not suspend UNIQUE indexes on either
+    // engine (#1162). A backup taken before migration 186 carries the
+    // duplicate photo rows that migration exists to remove, so batchInsert
+    // below would hit photos_event_external_relpath_uniq and roll the whole
+    // restore back — after every table had already been emptied. Drop it for
+    // the load and rebuild it once the rows are deduped, which is the same
+    // repair the migration performs.
+    let hadRelpathIndex = false;
+    if (await trx.schema.hasColumn('photos', 'external_relpath')) {
+      hadRelpathIndex = true;
+      await dropExternalRelpathIndex(trx);
+    }
+
     for (const table of tables) {
       await trx(table).del();
     }
@@ -384,6 +402,18 @@ async function replaceAllTables(tables, dataDir, currentAdmin, roleSnapshot, { c
       await trx.batchInsert(table, prepared, 100);
     }
 
+    // Restore the constraint the load ran without. Deduping first because the
+    // incoming rows may be exactly the duplicates migration 186 removes; the
+    // index creation then also proves the repair worked, inside the same
+    // transaction that would otherwise leave the target unprotected.
+    if (hadRelpathIndex) {
+      const removed = await dedupeExternalPhotos(trx);
+      if (removed) {
+        logger.info(`picpeakImport: removed ${removed} duplicate external photo row(s) from the archive (#1162)`);
+      }
+      await createExternalRelpathIndex(trx);
+    }
+
     const operatorId = await reinjectCurrentAdmin(trx, currentAdmin);
     if (operatorId && roleSnapshot) {
       await preserveOperatorRole(trx, operatorId, roleSnapshot);