how can i identify duplicates in Photos?

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 sizes, file names, sizes and capture dates.

These are the only apps that provided demo versions which could be tested on a test Photos library.

I also use PowerPhotos and PhotoSweeper.

Yes and no. Photos is checking for duplicates when you import photos and videos and will warn you, when you try to import the same item more than once. So you can avoid to import the same item twice. But it will not recognize duplicates if you convert the file to a different format or import an edited version, only identical files will be recognized.

There is no tool in Photos to scan for duplicates that already have been imported. You will have to search for them on your own or use a third party duplicate finder.

But not all duplicate finders are safe to use, see: Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support

Do not use any duplicate finder, that will delete photos from the Photos library directly. Use only tools, that collet the items in an album, so you can delete them using Photos.

I am using Photo Sweeper or PowerPhotos.