You will have to manually search for duplicate photos already stored in the library.
Launch Disk Drill and select Find Duplicates from the sidebar The Mac Photos app will not be able to help you find duplicate images that are already in your library, but it can automatically detect duplicates as you import them. Download and install Disk Drill onto your Mac. Scan Your PC or Mac for Duplicate Photos, Videos, Documents and Other Files Delete and Clean Up Duplicate Photos, Documents, MP3s, Videos, Emails and More So, if you make a mistake, you can restore the files back to their original location Home How to search for duplicate photos on Mac Photo Duplicate Finder for Mac - Gemini 2