How to clean up photo duplicates on Mac

macOS 14 Sonoma has a Duplicate finder that displays exact and similar pictures like this:

The illustration above shows two pictures that aren't exact duplicates-- they are different resolutions. The "Merge 2 items" option combines some information from the two pictures, chooses the "better" one, puts that one in all the albums that either is in, and removes the "lesser" one from the Library--relegating it to the Recently Deleted view. Or, if you want to make the choice yourself, you can select one and hit Delete.

The Duplicates view only appears when Photos finds duplicates to show you.

There are 3rd party apps that say they "clean up" your Mac-- many have been found to corrupt Photos files. Be very careful! Before Photos' duplicate finder was introduced, I had used the trusted 3rd party app PowerPhotos ($30) and  PhotoSweeper ($15) which is cheaper but doesn't have the range of Library utilities.