Can’t remove duplicates in Photos app on Mac mini

It is in your best interest to dump any "Cleaner" type apps that you may have used. They have a history of corrupting the Photos Library. I do hope you have a backup to your Library! If you are finding Photos not to be working as it is intended, that may be due to disruptive apps like these.

After getting rid of those apps, you may want to rebuild the Photos Library. Close Photos and re-launch by option-command-clicking the app icon. Or, if you use iCloud, a better plan would be to delete the Library so that Photos will make a new, empty one, and let it fill up form iCloud Photos.

NBla wrote: Thanks - I do have a backup in Time Machine. I’m going to restore the library, rebuild, and hope that the Duplicates folder works after that. 🤞🏼

Excellent! If you can, you should think about upgrading to Ventura. That will generate a whole new database, so it might fix problems like this. But certainly try there rebuild. Any corruption is likely in recent TM copies. But, and I'm not sure about this, exactly, Time Machine combines new data with the old stuff when restoring a Photos Library, so just restoring from TM might make it work better. So that's 3 things-- rebuild, Time Machine restore, and upgrade. Good Luck!