Duplicate Photos - macOS Catalina Photo Application

When iPhone and Mac are signed in with same Apple ID and password on the same network if a duplicate photo os created on any device it will select both ways https://support.apple.com/en-in/guide/icloud/mm290eb968fe/1.0/icloud/1.0

You must always back up photo library https://support.apple.com/en-in/guide/photos/pht6d60d10f/mac

And see the article to select photos https://support.apple.com/en-in/guide/photos/pht40b0be510/4.0/mac/10.14

If you use third party apps to delete duplicate photos see the article https://support.apple.com/en-in/HT204968

It will definitely corrupt the photo library and you have to repair the library https://support.apple.com/en-in/guide/photos/pht6be18f93/4.0/mac/10.14

But it doesn't mean start using third party apps to delete duplicate photos , repair tool might not work suppose the photo application gets corrupted , in the last creating new test user account may not work https://support.apple.com/en-in/HT204443

You have to erase the hard drive and reinstall Mac OS https://support.apple.com/en-in/HT204904

We're talking hundreds and hundreds of duplicates. It would take hours to go through an manually delete them all.

If you value your photos you should invest the few hours to handpick the keepers, or you may lose the only working version or high quality version of a photo.

Carry, try to find out common properties between the duplicates. It would be risky to delete them before you know, why they are there. Are you using iCloud Photos? Are you syncing photos Libraries with iCloud between macs with different system versions?

For example, in my library I found hundreds of duplicates.

• Many of them had been externally edited, and Photos seems to have unpaired the edited version from the original, so now I had the original and the edited versions. For these kind of duplicates you should keep the original. The edited version may be of lesser quality or incompatible with Catalina.
• Other duplicates have been caused by iCloud syncing - photos edited differently on different devices, for example on my Mojave Mac and the Catalina Mac, so Photos is showing both versions, and we have to decide, which version is the better version.
• Some photos have been recovered items, that have been lost and found in the library bundle during the upgrade to Catalina. These items have been saved directly to the Photos Library by external editors, without linking them to the originals, or lost during a crash. These items can be safely discarded.
• Some duplicates are previews of incompatible image files, that can no longer be processed by Catalina. In this case we need to export the originals and convert them to a compatible format.
• Some users reported that My Photo Stream is the reason for duplicates. For the time being, I would disable My Photo Stream, if you are using it.

Once you find out, what has been causing the duplicates in your library, and want to delete duplicates, I'd recommend to keep the original versions, not the edited versions, and always the file with the better resolution, the larger pixel size.Check the copy you decide to keep, if it can be opened in edit mode. You also have to check the metadata. Does the better version of the photo the correct capture date and are the titles and keywords applied?

In which view are you seeing the duplicates?

If it's in the Photos, Recent or Import views then consider 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  

The reason I can recommend these two apps is that they offered demo versions that I could test for reliability and safety. Those that don't offer a demo version I leave alone. I subsequently purchased both of the apps for use.

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.

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