Searching for duplicate images in Photos on Mac

The Photos library has a feature for identifying duplicate photos. The Duplicate album is located in the Utilities folder in Photos sidebar just below Featured Photos and just above Media Types:

It takes a while before it locates and displays the duplicates. Then you can merge them, adding whatever metadata is in each to the remaining photo.

If you want to be more pro-active and 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 - $40  

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 - $15 - 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.

I recommend PowerPhotos as it can do so much more like merging libraries, viewing photos in columns with the columns representing most metadata, size, etc. It is by far the best utility for Photos libraries.