It works and its free

This is a great little app to deal with one of the many annoyances you may have after moving from iPhoto to Photos.app
The only thing to be negative about is that the time to render the proposed duplicates is not zero. So it takes a bit of time to dig through a few hundred duplicates to make 100% sure you are not deleting something you want to keep. In my case this was unnecessary, the app auto marked all duplicates and no other pictures.

RokSci about Photos Duplicate Cleaner, v1.7