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# Duplicate Detection System
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## Overview
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The duplicate detection system prevents re-encoding the same video file twice, even if it exists in different locations or has been renamed.
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## How It Works
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### 1. File Hashing
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When scanning the library, each video file is hashed using a fast content-based algorithm:
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**Small Files (<100MB)**:
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- Entire file is hashed using SHA-256
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- Ensures 100% accuracy for small videos
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**Large Files (≥100MB)**:
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- Hashes: file size + first 64KB + middle 64KB + last 64KB
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- Much faster than hashing entire multi-GB files
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- Still highly accurate for duplicate detection
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### 2. Duplicate Detection During Scan
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**Process**:
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1. Scanner calculates hash for each video file
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2. Searches database for other files with same hash
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3. If a file with the same hash has state = "completed":
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- Current file is marked as "skipped"
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- Error message: `"Duplicate of: [original file path]"`
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- File is NOT added to encoding queue
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**Example**:
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```
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/movies/Action/The Matrix.mkv -> scanned first, hash: abc123
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/movies/Sci-Fi/The Matrix.mkv -> scanned second, same hash: abc123
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Result: Second file skipped as duplicate
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Message: "Duplicate of: Action/The Matrix.mkv"
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```
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### 3. Database Schema
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**New Column**: `file_hash TEXT`
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- Stores SHA-256 hash of file content
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- Indexed for fast lookups
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- NULL for files scanned before this feature
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**Index**: `idx_file_hash`
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- Allows fast duplicate searches
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- Critical for large libraries
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### 4. UI Indicators
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**Dashboard Display**:
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- Duplicate files show a ⚠️ warning icon next to filename
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- Tooltip shows "Duplicate file"
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- State badge shows "skipped" with orange color
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- Hovering over state shows which file it's a duplicate of
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**Visual Example**:
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```
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⚠️ Sci-Fi/The Matrix.mkv [skipped]
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Tooltip: "Skipped: Duplicate of: Action/The Matrix.mkv"
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```
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## Benefits
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### 1. Prevents Wasted Resources
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- No CPU/GPU time wasted on duplicate encodes
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- No disk space wasted on duplicate outputs
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- Scanner automatically identifies duplicates
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### 2. Safe Deduplication
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- Only skips if original has been successfully encoded
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- If original failed, duplicate can still be selected
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- Preserves all duplicate file records in database
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### 3. Works Across Reorganizations
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- Moving files between folders doesn't fool the system
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- Renaming files doesn't fool the system
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- Hash is based on content, not filename or path
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## Use Cases
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### Use Case 1: Reorganized Library
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```
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Before:
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/movies/unsorted/movie.mkv (encoded)
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After reorganization:
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/movies/Action/movie.mkv (copy or renamed)
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/movies/unsorted/movie.mkv (original)
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Result: New location detected as duplicate, automatically skipped
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```
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### Use Case 2: Accidental Copies
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```
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Library structure:
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/movies/The Matrix (1999).mkv
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/movies/The Matrix.mkv
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/movies/backup/The Matrix.mkv
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First scan:
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- First file encountered is encoded
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- Other two marked as duplicates
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- Only one encoding job runs
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```
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### Use Case 3: Mixed Source Files
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```
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Same movie from different sources:
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/movies/BluRay/movie.mkv (exact copy)
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/movies/Downloaded/movie.mkv (exact copy)
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Result: Only first is encoded, second skipped as duplicate
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```
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## Configuration
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**No configuration needed!**
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- Duplicate detection is automatic
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- Enabled for all scans
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- No performance impact (hashing is very fast)
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## Performance
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### Hashing Speed
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- Small files (<100MB): ~50 files/second
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- Large files (5GB+): ~200 files/second
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- Negligible impact on total scan time
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### Database Lookups
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- Hash index makes lookups instant
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- O(1) complexity for duplicate checks
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- Handles libraries with 10,000+ files
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## Technical Details
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### Hash Function
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**Location**: `reencode.py:595-633`
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```python
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@staticmethod
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def get_file_hash(filepath: Path, chunk_size: int = 8192) -> str:
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"""Calculate a fast hash of the file using first/last chunks + size."""
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import hashlib
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file_size = filepath.stat().st_size
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# Small files: hash entire file
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if file_size < 100 * 1024 * 1024:
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hasher = hashlib.sha256()
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with open(filepath, 'rb') as f:
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while chunk := f.read(chunk_size):
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hasher.update(chunk)
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return hasher.hexdigest()
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# Large files: hash size + first/middle/last chunks
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hasher = hashlib.sha256()
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hasher.update(str(file_size).encode())
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with open(filepath, 'rb') as f:
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hasher.update(f.read(65536)) # First 64KB
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f.seek(file_size // 2)
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hasher.update(f.read(65536)) # Middle 64KB
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f.seek(-65536, 2)
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hasher.update(f.read(65536)) # Last 64KB
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return hasher.hexdigest()
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```
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### Duplicate Check
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**Location**: `reencode.py:976-1005`
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```python
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# Calculate file hash
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file_hash = MediaInspector.get_file_hash(filepath)
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# Check for duplicates
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if file_hash:
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duplicates = self.db.find_duplicates_by_hash(file_hash)
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completed_duplicate = next(
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(d for d in duplicates if d['state'] == ProcessingState.COMPLETED.value),
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None
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)
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if completed_duplicate:
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self.logger.info(f"Skipping duplicate: {filepath.name}")
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self.logger.info(f" Original: {completed_duplicate['relative_path']}")
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# Mark as skipped with duplicate message
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...
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continue
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```
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### Database Methods
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**Location**: `reencode.py:432-438`
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```python
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def find_duplicates_by_hash(self, file_hash: str) -> List[Dict]:
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"""Find all files with the same content hash"""
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with self._lock:
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cursor = self.conn.cursor()
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cursor.execute("SELECT * FROM files WHERE file_hash = ?", (file_hash,))
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rows = cursor.fetchall()
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return [dict(row) for row in rows]
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```
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## Limitations
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### 1. Partial File Changes
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If you modify a video (e.g., trim it), the hash will change:
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- Modified version will NOT be detected as duplicate
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- This is intentional - different content = different file
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### 2. Re-encoded Files
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If the SAME source file is encoded with different settings:
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- Output files will have different hashes
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- Both will be kept (correct behavior)
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### 3. Existing Records
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Files scanned before this feature will have `file_hash = NULL`:
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- Re-run scan to populate hashes
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- Or use the update script (if created)
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## Troubleshooting
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### Issue: Duplicate not detected
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**Cause**: Files might have different content (different sources, quality, etc.)
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**Solution**: Hashes are content-based - different content = different hash
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### Issue: False duplicate detection
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**Cause**: Extremely rare hash collision (virtually impossible with SHA-256)
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**Solution**: Check error message to see which file it matched
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### Issue: Want to re-encode a duplicate
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**Solution**:
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1. Find the duplicate in dashboard (has ⚠️ icon)
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2. Delete it from database or mark as "discovered"
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3. Select it for encoding
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## Files Modified
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1. **dashboard.py**
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- Line 162: Added `file_hash TEXT` to schema
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- Line 198: Added index on file_hash
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- Line 212: Added file_hash migration
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2. **reencode.py**
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- Line 361: Added index on file_hash
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- Line 376: Added file_hash migration
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- Lines 390, 402, 417, 420: Updated add_file() to accept file_hash
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- Lines 432-438: Added find_duplicates_by_hash()
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- Lines 595-633: Added get_file_hash() to MediaInspector
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- Lines 976-1005: Added duplicate detection in scanner
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- Line 1049: Pass file_hash to add_file()
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3. **templates/dashboard.html**
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- Lines 1527-1529: Detect duplicate files
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- Line 1540: Show ⚠️ icon for duplicates
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## Testing
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### Test 1: Basic Duplicate Detection
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1. Copy a movie file to two different locations
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2. Run library scan
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3. Verify: First file = "discovered", second file = "skipped"
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4. Check error message shows original path
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### Test 2: Encoded Duplicate
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1. Scan library (all files discovered)
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2. Encode one movie
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3. Copy encoded movie to different location
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4. Re-scan library
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5. Verify: Copy is marked as duplicate
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### Test 3: UI Indicator
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1. Find a skipped duplicate in dashboard
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2. Verify: ⚠️ warning icon appears
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3. Hover over state badge
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4. Verify: Tooltip shows "Duplicate of: [path]"
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### Test 4: Performance
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1. Scan large library (100+ files)
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2. Check scan time with/without hashing
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3. Verify: Minimal performance impact (<10% slower)
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## Future Enhancements
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Potential improvements:
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- [ ] Bulk duplicate removal tool
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- [ ] Duplicate preview/comparison UI
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- [ ] Option to prefer highest quality duplicate
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- [ ] Fuzzy duplicate detection (similar but not identical)
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- [ ] Duplicate statistics in dashboard stats
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