These days of massive music collections it's easy to end up having the same song twice, or even three times. The issue is that, not only does this make you waste a lot of valuable space in your hard drive, but it also bothers you when trying to create playlists avoiding repeated files.
In order to solve this problem you need a tool like Music Content Inspector. Under such a serious name you'll find a program that analyzes your music folders deeply, searching for and displaying all duplicate files.
You can play them from the program itself (just to make sure they're the same) and then delete one of the copies.
Results are saved in a database file so they can be accessed later on.