I have seen this in an old version of OpenLP. My first attempt to solve it was TOOLS > Songs > Re-index Songs. Then run the find duplicates.
You may need to delete the song from database. Then load the service with "Import missing songs" set to load the song back into the database.
This would happen when we were trying to run OpenLP installed on different systems. Carrying a service from one to another should have been fine but, our songs would get out-of-wack. We went to using a portable version of OpenLP on a USB drive with its associated data files.
Fred