duplicate photos?

Welcome to the Apple Community.

Unfortunately at this time there are no tools within Photos to search for and remove duplicates and no suitable workarounds to the problem.

You might try this third party solution. There may be other 3rd party solutions.

Isn't it ironic, Winston Churchill, that the Photos.app in Catalina is taking an awful lot of time searching for duplicates, but it is keeping the result secret from the user. It is using the duplicate detection just to hide the duplicates from the curated view in the Photos > Day, sweeping them below the carpet out if sight, so it is even harder to get rid of duplicates. 😀

Since I first installed Catalina, Photos is cycling between "Curating best Photos, Analysing, scenes, detecting duplicates", but it is hard to find any use for this effort.

The best we can do to profit from this, would be to create an album from the photos that have been omitted from the curated view and use them as candidates for deletion.

About the Duplicate Detection in Photos 5 - Apple Community

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, both original and edited versions, 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'd never actually seen it doing this on my library, I'd known about the feature, but wasn't sure what it was for since I hadn't seen it.

You will only seeing it, if the Mac is connected to power, if you are using a portable Mac. It appears below the Days. When I pull the plug and the MacBook Pro is using the battery, I am only seeing this message for my large main library:

or this for the smaller test libraries, where Photos managed to to finish with the curation:

But my main library never seems to finish the curation cycle. It does not really matter for me, because I rarely look at the Days, Months, Years views. I want to see my photos in the correct aspect ratio, with titles and badges, and all of them, not the ones Photos has selected as my best photos, because my taste seems to be very different from Photos's taste.

I have successfully tackled multiple duplicates (my own fault!) using PowerPhotos. The paid version (good value) will identify duplicates, allow you to choose which to delete as well as merging libraries, copying between libraries and so on. For me, it works better than Photos in handling Photos Libraries.

Stephen

I'd never actually seen it doing this on my library, I'd known about the feature, but wasn't sure what it was for since I hadn't seen it.

Now that you've mentioned it and linked to your previous discussion, I've had chance to have another look. If I quit photos and open it again, I do get a brief message about curating at the bottom of the screen, but it's never there long enough to read the whole sentence.

That makes sense, thanks.