Does iPhoto mark duplicates

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.

Photos 5 on Catalina is spending a lot of time searching for duplicates, but it is keeping them secret and does not show them to us for removal. See: About the Duplicate Detection in Photos 5 - Apple Community

Photos is just searching for duplicates to present us the library with a less cluttered view.

Be careful, when you are selecting a third-party app for duplicate detection. Some are dangerous and can damage the Photos Library badly. Do not use any application that will delete the photos directly from the library. Apple is warning against such software:

Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support

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 successful 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.

Not by itself, no.

Some apps for dealing with duplicates in iPhoto

iPhoto Library Manager

Duplicate Annihilator

Keith Barkley wrote:

That's fine, Leonie, but Duke44 seems to be using iPhoto. 8^)

The system version given below the original post does not support iPhoto, Keith, so at least one of his Macs must be running Photos.

That's fine, Leonie, but Duke44 seems to be using iPhoto. 8^)