How to find and delete duplicate photos in the "Photos app"?

Depending on the size of your library and how long you've had it in the current Photos version it can take a while before duplicates show up. So, as they say, patience is a virtue.

If you want to search from them yourself the following may be of interest to you:

You want an app that will identify the potential duplicates, put them in an album or mark them with a keyword for easy retrieval and deletion by you. You don't want one that does the deletion itself for obvious reasons. 

I've run tests on the these two apps with the following results and found them to be safe to use:

PowerPhotos - $29.95  

PowerPhotos is the iPhoto Library Manager version for Photos and is very powerful. Although more expensive I would recommend it as it has more capabilities than the others like the capability to merge Photos libraries or copy photos, both original and edited versions, along with their metadata between libraries.

PhotoSweeper - $9.99 - Demo version available.

PhotoSweeper compares bitmaps and/or histograms so it can detect duplicate images even if they have different file sizes, file names, image sizes and capture dates.

Just some food for thought.

In Mac Photos, there may be a "Duplicates" folder, like this:

The Duplicates Folder only shows up when Photos has found duplicates to show you. It can take quite a long time for Photos to find these. Finding duplicates is hard, and your Mac and Photos are busy doing lots of things. It's best to be patient for for this.

I see that I have 17 duplicates. I understand that some of those hang around because the are very similar, but they're not exact duplicates, and I want to keep both. Some just showed up, so I'll take a closer look. The two above look the same, but one is a bigger file than the other, so I might want to merge them. I don't blindly erase all of the duplicates it finds, because there are some like these that are just similar, and sometimes I have a reason to keep them both.