Eliminate Duplicate Photos
Is there an automatic way to delete duplicate photos?
iMac 21.5", macOS 10.13
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On macOS 10.13 you would need a third-party application to detect duplicate photos, but not all are safe to use and can damage the library badly. Apple is warning against them.
Using third-party apps to remove duplicate photos might damage your Photos for macOS library.
Only duplicate detectors, that do not modify your library directly are safe. Try to find one, that is collecting the duplicates in an album for you to delete using Photos.
I find Power Photos reliable, also Photo Sweeper.