Media players are showing duplicate files after moving them
The music players get their information from the central Android Media Storage database (part of the Android OS), therefore both players were showing the same information.
When moving a large number of files around you have to keep calm and wait until the database has updated/re-scanned all entries. This happend automatically in background and can take some time depending on the number of files moved.
If after a few hours the situation is still not correct reboot the device, that should trigger a full re-scan of all media files.