On this video, Laura Shoe takes a look at a plug-in which allows you to (relatively) easily find and remove duplicate images in your Lightroom collections.
The Duplicate Finder is a useful piece of software which was created by Jim Keir. What you need to remember is that this plugin only work with images in your Lightroom collections and not any image on your computer (there are other programs that can do that – like the free AuslogicsDuplicate File Finder – see a review here).
Shoe looks at some real world examples of using the plugin althogh she doesn’t go over all the features that the software has (including using EXIF data in some smart ways). As Keir explains it in his website:
There are already utilities which will compare files to see if they’re identical, but they don’t cope well with Lightroom images. Because Lightroom can write metadata back into the files, identical photos can be different on disk because of something as simple as a ‘last edit time’ value. They also don’t help where one image has been changed, for example scaled, but is still essentially the same image. By using the EXIF data, this plugin finds all photos which came from the same source; it compares the camera model, serial number, lens, ISO speed etc. It can also filter by keywords and filename to narrow results down further.
Official installation guide for the duplicate finder plug-in
[youtube]https://www.youtube.com/watch?v=kgNfx2Auaio[/youtube]
You can download the software for about $12.5 from Keir’s website.
You can check out more useful Lightroom guides and tips on our LensVid Lightroom section. You can watch more of Shoe’s video tutorials published on our site here.
Iddo Genuth 
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Nice tutorial, but this works better in Windows: https://www.mindgems.com/article/lightroom-duplicate-finder-delete-duplicate-photos/
and has much more features.