It works. Try not to overthink it is my advice.

The gist: IF you properly back up your photo library (i.e., have Time Machine spinning a backup to a disk and a second backup somewhere), THEN letting this baby spin on your duplicates brings no peril. Just do it. It took me less than 20 minutes to remove about 8000 files and restore nearly 8 Gb of storage space in a Photos library.

I “auto marked” all of the duplicates after going to Preferences and setting auto-mark prefs to mark these images: _non-library photos; _smaller DPI; _smaller size; _images without metaData. Remember, its applying this logic to DUPLICATES and how to choose which to mark.

Then I deleted (app walks you through the instructions of what is about to happen and what to do next). Then went into the Duplicates Marked … folder, deleted them. Then went straight to recently deleted items and deleted them for good. Done.

Background if you care...
My wife’s frustration with keeping photos straight through the variety of changes Apple has made to how they integrate photo libraries and devices over the last 10 years has been getting higher and higher. Alas on Photos she had about 20 Gb in her library, many of which she knew were duplicates, mostly caused by re-syncing devices (phone or iPad) on which photos had been deleted and then importing them again into Photos library.

I chose this app because it sounded from the reviews like it just worked. And it does. Reading one review on ads, I was willing to buy the Pro version (for $3) but couldn’t see any real value in doing that (in fact, I couldn’t tell what it would do that this didn’t do).

THIS version is for Photos - not iPhotos. In their “Help” files they point this out and point to a version of this app for iPhotos (lots of reviewers complaining about the instructions and about the lack of iPhotos support - guess I don’t understand the complaint).

It works.

Thisgirl3476 about Photos Duplicate Cleaner, v1.7