Which duplicate photo removal app is best for iMac?

If you're running Ventura or newer there's a duplicate finding feature in Photos. It usually takes a while to can all of the photos for duplicates and display the Duplicates folder under Utilities section is Photos sidebar.

If you want to be proactive in the duplicate remove process consider the following: 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 these two apps with the following results and found them to be safe to use:

PowerPhotos - $40  

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 - $15 - 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.

Actually PowerPhotos does more that just find duplicates. It is a way of viewing photos in a library without opening Photos. And one can use a column move to view and sort on a number of image metadata and EXIf metadata which Photos can't as well as merging libraries more completely than Photos can. . It's my preference.

We all probably have a lot to say about Duplicates! Continuing from Matti's and Old Toad's excellent comments:

The most important thing to know here, as Old Toad has said, is that many 3rd party apps claiming to "clean up your" device may in fact do more damage than good. You should be very careful! The two that Old Toad suggested are known to be safe, though I'm not sure about others. Really, you should have a good backup (don't rely on iCloud Photos for this!) before you start a 3rd party duplicate finder.

Photos' own Duplicate Finder is excellent.  Photos must scan through all the pictures in the Library, and it doesn't see this has as high priority in light of the battery charge and bandwidth required. So it may take some time for the Duplicates search to complete. As with other background operations, it's best to find good wifi, keep the Mac turned on, don't have Photos in the foreground, and leave it alone. If the Mac thinks that you're busy using Photos then it won't do scanning that might slow you down.

Be careful, though: In Photos on a Mac there is this warning at the bottom of the Duplicates page: 

(This warning appears under an inconspicuous (i) symbol on an iPad or iPhone.) It's wise to pay attention! Some pictures look alike, but aren't, like this pair: 

I would be sad to accidentally delete one of these.

There may be lots of differences even between "exact" duplicates. They may have different metadata, for instance, that you don't expect. I always compare the Info Windows of two pictures, and before merging or deleting I copy data from one and add it to the other. 

The one great thing that merging does is keep the album organization. If one picture is in the Aunt Ethel album, and the other is in the Grand Canyon album, then Merge will delete one and put the other in both albums. 

I don't always agree with Photo's choice of the best picture to keep. Luckily, merging puts the "loser" picture in Recently Deleted, so you may want to scan through those before you allow them to be automatically erased in 30 days. 

After you've ensured that you agree with the choices, you can select them all, or select a swath of them using a shift click, and choose Merge-- you don't have to do it two at a time. You can also just choose 5 pairs, for instance, or any number at a time. Using Command-click, you can choose non-contiguous groups.

whdkc wrote:

My question is which duplicate removal application would be best.

...another question is which "duplicate" is the best.

I would not blindly trust any app to delete "duplicates". Some "duplicate" might be visually the best or have most useful metadata.

...I maintain a shared album of historical houses and places with added metadata (Dates, Descriptions, Keywords, and GPS, I add Description metadata who shot the image to the best of my knowledge) currently via Google Photos although it is far from perfect).

I collect such old images and movies from many sources and archive originals in their original format. If needed, I sometimes losslessly crop them for sharing.

Very often there are multiple duplicate images with different codec and format (.jpg, .heif, .tif, .avif, .wepb, .gif, .pdf, documents..., movies... etc), resolution, crop etc.

Then I have to visually check which one has the best quality for sharing.

Largest file size or resolution is not always the best quality because often those images are re-saved with different compression or upscaled resolution with no real visual benefit. So I always zoom new images to check if they are any better compared to the best old one(s).

But often I have to include several different crops each displaying more interesting area.

Are the photos in the photos app? There is a built in duplicate finder.

otherwise, you can try a myriad of apps that find duplicates. Best is a relative term, so you’ll want to investígate options.