Powerphotos - PowerPhotos lets you organize your Photos libraries on the Mac. You can split a library into smaller ones, merge libraries together, find duplicate photos, and more.
dupeGuru - dupeGuru is a tool for finding duplicate files on your computer.
Monument Backup and Organization Device - Effortlessly store and manage all of your photos.
AllDup - AllDup is a freeware tool for searching and removing file duplicates on your Windows computer.
digiKam - Professional Photo Management with the Power of Open Source
Czkawka - Multi functional app to find duplicates, empty folders, similar images etc.