Duplicate Photos on Mac

I recommend Gemini from MacPaw.

Search photos see this article  https://support.apple.com/en-in/HT207368

Using third party apps / softwares to find duplicate photos can damage photo library https://support.apple.com/en-in/HT204968

You can manually delete duplicate photos see the thread https://discussions.apple.com/thread/250983357

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 following apps with these results::

PhotoSweeper - $9.99 - Demo version available. 

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 along with their metadata between libraries.

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 tested the Gemini app on a couple of test libraries on my iMac that had 10 -20 known duplicates and it found absolutely none. MacPaw also offers CleanMyMac which is considered akin to malware by most around here. Personally I wouldn't run any of their apps on my Mac. I ran it this time on a backup system.