How do I remove duplicate photos using High Sierra?

Photos on High Sierra does not have a duplicate finder for photos, once the photos have been imported into the library. It is warning about dulicates, when you import photos that already have been imported. At thetime of import it will check for identical image files already in the library.

If you managed to import duplicates nevertheless, you have to screen the library manually for duplicates by sorting the photos by the capture date or use third-party duplicate finders. Nt all are safe to use, however (see: Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support).

These duplicate finders are safe, because they do not modify the library directly, but collect duplicates in an album:

• Power Photos
• Photo Sweeper
• Duplicate Annihilator

There may be more, but I did not test them.

Power Photos is very versatile as a second browser for your Photos Library. It can display the library with metadata below the thumbnails or in a sorted list view according to the metadata. It will detect duplicates according to the file attributes and the capture date.

Photo sweeper can detect similar photos, not only exact duplicates. If you have several edited versions of the same photo, Photo Sweeper can find it.

Same as with all previous versions of Photos - use safe, tested duplicate identification software like

PowerPhotos

Duplicate annihaliator for PHotos

PhotoSweeper for Photos

Do not use malwear like Gemini, CleanMyMac or MacKeeper - they are very dangerous and may destroy your Photos library and even damage your Mac requiring totally wiping it and starting over with a clean, fresh install

LN