The main one is that you have an automatic import happening (iCloud Photo Library, for example), and then you trigger a manual import. Typically duplicate photos accumulated due to a couple of reasons. So, duplicate photos are a common problem on Macs. Photos may not be able to detect if it already has those photos in the library yet but doing it during the import has proven to be a reliable solution so just be prepared to do a massive import for now. Such as you imported them from mobile phones or downloaded them from the internet. (4,3000 photos was a little stuttery on my 2012 Mac mini i5 but it did work after being patient) Be patient with it and let it do it's thing but it is working. Notes: Since you mentioned you have 5000 photos, the Mac will potentially become a little unresponsive while you're dragging them into the window. There you have a full library of purely unique photos.During the import you will be prompted to choose what you want to have done for duplicate photos and it will show you the first two identical photos that it has found, select 'skip' and 'apply to all'.Open that new empty library in photos and drag everything (using CMD+a in finder) in that exported folder to the Photos.app window.Close photos and create a new photos library.

Export your entire photo library to a folder.All 3 of my devices now have duplicates of everything. I suspect, but don’t actually know, that a photo on your Mac that you’ve previously deleted from iCloud Photo Library by deleting from your iPhone is going to get uploaded again. It says I have like 4000+ photos which some I am sure I deleted long ago, but whatever. In theory, it will detect all the duplicates on your Mac and not upload them. Never used it before, but decided to try it out. Here's what I did to remove the duplicates quickly and easily. Ok Yesterday I turned on iCloud photo on all 3 of my devices - my iMac, iPhone X, and iPad Pro.

Duplicate photos mac icloud