Duplicate files in the library is usually caused by having multiple folders in the scan paths pointing to the same folder. sdcards are not case sensitive, so listing /storage/sdcard/Music and /storage/sdcard/music will point to the same location but there is no way for GMMP to know they are the same.
How to fix:
- Ensure scanner settings -> edit scan paths is only pointing to your music folders and you dont have any duplicate entries like mentioned above.
- Delete the database under the advanced scanner settings
- Run a new scan