Removing duplicate photos
How do I find and remove duplicated photos
MacBook Pro
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Photos has no tool to show us duplicate image files. We have to search for them ourselves, or use third-party duplicate Finder.
It helps to view the photos in an album, where they are sorted by the date the photo has been taken and all photos are shown. On macOS 10.15 I would view them in the All Photos album. Then duplicate files will be shown side.by-side.
Photos 5 on Catalina is spending a lot of time searching for duplicates, but it is keeping them secret and does not show them to us for removal. See: About the Duplicate Detection in Photos 5 - Apple Community
Photos is just searching for duplicates to present us the library with a less cluttered view.
Be careful, when you are selecting a third-party app for duplicate detection. Some are dangerous and can damage the Photos Library badly. Do not use any application that will delete the photos directly from the library. Apple is warning against such software:
From my own tests I can recommend PowerPhotos as safe, also Photo Sweeper.