Duplicates Utility in Photos App.

hughbin wrote: … when I click on "Duplicates", nothing happens and I get a message appearing that says there are "zero" duplicates.

That's weird-- if you have no duplicates, then the "Duplicates" view does not appear. While my usual Libraries have plenty of duplicates, I just checked a temporary Library I had made some time ago with no Duplicates, and the Duplicates view isn't there. (I checked this to make sure it hasn't changed in Sequoia.)

The first things to try are these:

• Restart the computer (of course)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes. Safe Mode is different for different computers, so see this: 

        Start up your Mac in safe mode - Apple Support 

Those are the most important things to try. Then try these:

• Make a new user and see if the same problem recurs with it
• Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
• Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon.

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.