diff --git a/backend/app/tasks/scan.py b/backend/app/tasks/scan.py
index 71c1760..97efa54 100644
--- a/backend/app/tasks/scan.py
+++ b/backend/app/tasks/scan.py
@@ -11,7 +11,7 @@ import json
 from typing import List, Dict, Optional
 
 from celery import shared_task
-from sqlalchemy import select
+from sqlalchemy import select, func
 from sqlalchemy.ext.asyncio import AsyncSession
 import aiofiles
 import redis
@@ -172,10 +172,21 @@ async def _scan_folder_async(folder_path: str, source_root_id: Optional[str], ta
                             # Calculate file hash for duplicate detection
                             file_hash = await calculate_file_hash(filepath)
 
-                            # Check for duplicate by hash
-                            duplicate = await session.execute(
-                                select(Photo).where(Photo.file_hash == file_hash)
-                            ) if file_hash else None
+                            # Check for duplicate by hash. We only care
+                            # whether *any* other photo shares this hash, so
+                            # use a count rather than scalar_one_or_none()
+                            # which raises "Multiple rows were found" the
+                            # moment the library has 2+ copies of the same
+                            # file (i.e. exactly the case we're trying to
+                            # flag).
+                            is_dup = False
+                            if file_hash:
+                                dup_count = (await session.execute(
+                                    select(func.count(Photo.id)).where(
+                                        Photo.file_hash == file_hash
+                                    )
+                                )).scalar() or 0
+                                is_dup = dup_count > 0
 
                             # Create photo entry
                             photo = Photo(
@@ -188,7 +199,7 @@ async def _scan_folder_async(folder_path: str, source_root_id: Optional[str], ta
                                 file_size=stat.st_size,
                                 taken_at=datetime.fromtimestamp(stat.st_mtime),
                                 taken_at_source='filesystem',
-                                is_duplicate=bool(duplicate.scalar_one_or_none() if duplicate else False),
+                                is_duplicate=is_dup,
                                 processing_status='pending'
                             )