Duplicating photos has stopped working?

A few things to check immediately.

If the library is on an external drive, is it formatted correctly (APFS or MacOS extended (journaled)

Try starting in safe mode - does it work now - if so, restart normally - is it fixed?

Try creating a new library - add a few photos. Does it work in the new library?

Try a new user account - does it work in a new user account?

I had the same problem.

if you try and duplicate a batch of photos and one fails then the whole batch fails. So firstly try and only select a small number of photos at a time.

I found that if you open each photo in edit mode and make a small edit adjustment or crop to it when you return to the library view you should be able to duplicate that photo. It’s a hassle but you can work through a batch of photos quite quickly.