Q: Duplicate Photos.

Is there a way to automatically delete any duplicate photos from the Photos app?

Are you asking about iPhoto (you are posting in the iPhoto forum) or about the Photos for Mac application?

But the answer is the same in both cases. Neither iPhoto nor Photos for Mac has a built in tool to search the library for duplicates. Both applications are searching for duplicates when you import photos and alert you about potential duplicates on import.  If you import the duplicates nevertheless, you are on your own.

There are third-party duplicate detectors that are safe to use: 

• iPhoto Library Manager for iPhoto, and the successor to iPhoto Library Manager "Power Photos" for Photos for Mac
• Photo Sweeper
• Duplicate Annihilator

Don't use cleaning applications that modify the photo library directly, that are not safe.

Which app to use will depend on the kind of duplicates you need to find.  Duplicate Annihilator and Power Photos or iPhoto Library Manager are very good at finding exact duplicates based on the file attributes. Photo Sweeper is comparing the photos based on histograms or pixel maps and can detect edited versions of the same photo or duplicates  in different file sizes.

For Photos for Mac the best options are Power Photos (see the web page: https://www.fatcatsoftware.com/powerphotos/)  and Photo Sweeper: https://itunes.apple.com/en/app/photosweeper/id463362050?mt=12

Thanks for this recommendations, I have the same question (Photos for macOS Sierra).  But I find that the Photos Import process does not in fact warn me about duplicate photos, at least based on the photo content itself.  I just went through and imported a lot of old iPhone camera uploads (.jpg) from a few years ago, before I used iPhoto or Photos.  There were duplicates in some of the uploads because the photos had not been deleted from the iPhone between uploads.  The filenames were different but the photos were identical to some I had already imported and placed in albums.  Photos did not seem to detect the duplicates. So:

• Does Photos look at content or just metadata when looking for duplicates ?
• Does Photos look for duplicates within the current Import only, or does it compare the current import to the whole library ?

Photos is looking for identical files, not identical content. The file attributes have to be the same.

If you try to import a file you already imported into your library, you should be seeing a warning like this:

If you need to screen the Photos library for duplicate images (scaled down versions or edited versions of the same image), the best app would be Photo Sweeper. It can compare the content of a photo based on histograms or bitmaps and detect duplicates, if the file attributes are different.