Worked fine… once.

The first time I ran it, it found a duplicate that I knew I already had in my Photos library. The automated process to move the duplicate to its own album for ease of deleting it worked great. I deleted the duplicate. So far, so good.

Then I imported a few more pictures into Photos that I knew were duplicates. When I ran Photos Duplicate Cleaner again, not only did it not find the new duplicates, it kept reporting the one that I had deleted the first time. I tried a number of things — restarting Photos Duplicate Cleaner, removing then readding my Photos library from it, removing the empty duplicate photos album from Photos — but nothing made a difference.

Craig S. Cottingham about Photos Duplicate Cleaner, v1.7