If you have a huge Photos library on the Mac, you very likely have a bunch of duplicate photos. People have also found that duplicates photos are sometimes created when you upgrade from iPhoto or Aperture to Photos. Unfortunately, if you look at the “free” programs on the App Store, you find that they are generally either scams or teasers for a paid version.
So, I wrote an AppleScript to go through every photo selected in Photos (you can select your entire library easily, if you want) and create a new Album with just the duplicate photos found. The script can be copied to your Photos script folder (if you have the script menu enabled) or you can just run it from with ScriptEditor. I also created a small script to reveal the ID of a selected photo, which can be useful for debugging the library and/or finding an original file within the inscrutable Photos database. Most people (if anybody) will just want the Find Duplicates script. Download them at the link below:
Once the script finds any duplicates, it creates a folder called “Duplicates” and adds a new album with the found duplicates. You should then go through that album and confirm that the duplicates are legitimate. If they are, select which the one duplicate you want to remove (you won’t want to get rid of the entire pair, presumably), right click and select delete. Make sure you don’t delete both in the pair, but if you do it’s not the end of the world; they will stay in the deleted photos album for a month.
Let me know if something doesn’t work right. The Photos AppleScript library doesn’t allow any destructive operations other than deleting albums (which I don’t do in either script) so the worst case should be that if it doesn’t work it just doesn’t find duplicates that it should.
Update: As a couple folks pointed out in the comments, the next step will be to create a script that allows you to “merge” a pair of photos, moving all the album associations and tags onto just one. I haven’t looked into whether this can even be accomplished with the Apple Script hooks available in Photos, so if anybody has an insight I’d appreciate it.