Duplicate photo finder

Apple is warning against duplicate finders: Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support

Use only duplicate Finders that do not erase duplicates directly by modifying the Photos Library, but collect the duplicates in an album, so you can use Photos to erase them.

I found Power Photos to be safe, also Photo Sweeper. There may be others, but I have not tested them.

Apple doesn't encourage the use of duplicate photo finder software as some third-party apps may damage the Photos library. But it's time consuming and tedious to find and remove duplicate photos by hand. I use Cisdem Duplicate Finder for Mac. This app can efficiently detect and delete duplicate & similar images, duplicate audios, videos, documents, archives, and other types of duplicates on Mac's internal storage, external HD, SSD, USB flash drive and other storage devices. For more information, you can go visit its official website.