duplicate photos on Photos 2.0

Photos does not have a built-in tool to search for duplicates. It relieson the duplicate detection, when you import photos to Photos.

If you import duplicate photos nevertheless and ignore the duplicate warning, Photos will add them to the library.

Once the duplicates are in your library, you are on your own to track them down.

You could use third-party applications to search for duplicates, but be careful, which you use. Only some of them are safe to use. If the app is modifying your Photos Library and not just collecting the duplicates in an album for you to move to the Recently Deleted album, the app may destroy your library beyond repair. See Apple's warning: Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support

Some third-party apps for your Mac that claim to clean, shrink, de-clutter, or remove duplicates from Photos can instead damage your photo library or delete pictures and related data.

I found these three duplicate detectors safe to use. They are working well on my Photos library, even with iCloud, but have not tested all available third party apps.

• Power Photos
• Photo Sweeper
• Duplicate Annihilator

To add to léonie's recommendations regarding duplicate image finders the following are the apps that I've been able to test on actual Photo libraries:

These applications will identify and help remove duplicate photos from a Photos Library:

• Duplicate Annihilator for Photos - $7.95
• PhotoSweeper - $9.99 - Demo version available.
• PowerPhotos - $29.95
• Photos Duplicate Cleaner - Free

PowerPhotos is the iPhoto Library Manager version for Photos and is very powerful. Although more expensive I would recommend it as it has more capabilities than the others like the capability to merge Photos libraries or copy photos along with their metadata between libraries.

PhotoSweeper compares bitmaps and/or histograms so it can detect duplicate images even if they have different pixel sizes, file names and capture dates.