In the database maintenance screen there is an option to check for duplicates. It is called twin-checker. You have a few options on how strict the checking is, because many times the headers will be slightly different but the games are identical.
You'll have to decide what you want to do with the duplicates and if you mark them for deletion you will still have to compact the DB to remove the games.
in scid vs pc,when we download databases like millbase, gorgonianbase, etc, there is all posibility that the databases contain duplicates. in that case how can we find out the duplicates and delete them?