How do I identify and clean up duplicates in Photos

If you're running Ventjura and Photos V8.0 a duplicate finding feature is built into Photos. It may take a couple of days for the Duplicates folder to appear in the sidebar as Photos has to finish its search for faces, objects, etc.

If you're not running Ventura then there are apps 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.

I personally prefer PowerPHotos. It's the best 3rd party companion to a Photos library there is.