Gemini from MacPaw is a best of class duplicate file finder. There are quite a few apps that have the ability to find duplicate files, but Gemini is the best I've come across.
Gemini can start its search from any point. For example, you can select your Home folder and look for duplicates, or select two or more folders, and Gemini will look for duplicates in each set of folders. When it finishes searching, Gemini displays the duplicates in a list organized by type, including Music, Movies, Folders, Pictures, Documents, and Archives. I found the organization of the duplicate list particularly helpful. I was surprised to discover that I have two copies of a video taking up about 4 GB of storage. Gemini found almost 15 GB in duplicate files, a significant waste of storage space.
Gemini also helps you decide which files to keep and which to give the heave-ho, which is also a particularly useful feature. Select a file and Gemini will display all of the duplicates, plus their pathnames and modification dates. The program even adds a few notes about the files. For instance, Gemini told me that although I have three duplicate photos of a Morgan horse, it can't remove one of the duplicates because it's contained within an Aperture image library.
Removing duplicates is as easy as clicking a check box; you can also set Gemini to remove duplicates automatically. I prefer to make the decision, and I recommend that you do the same. In the example of the Morgan horse photos, Gemini's auto selection would have deleted the original masters, since they were the oldest versions; that isn't what I would have chosen to do. Despite some differences of opinion, Gemini is my go-to app for finding and removing duplicate files.
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Gemini is $4.99. A demo is available.