The 'duplicate photo' function in iPhoto no longer works. (Using MacBook Air)
In iPhotos on my MacBook Air the 'duplicate photo' function no longer works. Any advice?
MacBook Air 13″, macOS 10.15
If you're using Mac OS10.15 are you sure you're using iPhoto? It sounds more like you're running the Photos app that replaced iPhoto 6+ years ago.
Try restart your machine and see if it works after.