Duplicate Photos in Apple Photo Library

It is risky to use an app that will delete duplicates automatically. Even if the filenames are identical, the files might be different. For example, the image numbers created by your camera will wrap around after ten thousand or a-hundred thousand photos, two cameras may be using the same filenames. You can use duplicate detectors like PowerPhotos to point out potential duplicates and suggest the keepers, but check the files, that have been picked as keepers and compare them with the candidates for deletion. Keep the version with the largest dimensions in width and height, keep the original files as well as the edited version, keep the files you have tagged with titles and keywords or used in projects.

Power Photos is one of the few duplicate detectors that will not damage your Photos Library, because it is not modifying the library behind Photos’s back; Apple is warning against duplicate detectors: Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support

Try to find out, why you are having duplicates in your Photos Library. Photos is checking for duplicates when you import photos, so there should not be any exact duplicates, only similar photos - edited versions of the same photo, downsized versions downloaded from My Photo Stream or Shared albums, versions converted to a different file format. The most common reason for duplicates is mixing two methods of importing photos, for example My Photo Stream and manual importing, or iCloud Photos and manual importing, or the format conversion when downloading from an iPhone.

Once you know what is creating the duplicates you can try to avoid it or use it to search for the duplicates systematically.