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Duplicate files are a waste of disk space, consuming that precious SSD space on a modern Mac and cluttering your Time Machine backups. Remove them to free up space on your Mac.
There are many polished Mac apps for this -- but they're mostly paid software. Those shiny apps in the Mac app store will probably work well, but we have some good options if you don't want to whip out your credit card.
Gemini and Other Paid Apps
If you do want to spend money on a duplicate-file-finder app, Gemini looks like one of the best options with the slickest interfaces. The trial version worked well for us, and the interface certainly stands out from barebones, free applications like dupeGuru. Gemini can also scan your iTunes and iPhoto library for duplicates. If you're willing to pay $10 for a better interface, Gemini seems like a good bet.
There are other, similarly polished duplicate-file-finders in the Mac App Store, too -- but Apple flags this one as an Editors' Choice, and we can see why.
As a bonus, the demo version of Gemini allows you to search for and find duplicates, but not remove them. So, if you really wanted, you could use the demo to find duplicates on your Mac, locate them in Finder, and then remove them by hand. Other paid duplicate-file-finder apps have demos that function in a similar way, so this may be convenient if you just want to run an occasional scan and you don't mind deleting a handful of duplicates by hand.
There are many good-quality, paid duplicate-file-finding apps for Mac. You can find them with a quick trip to the Mac App Store.
dupeGuru, dupeGuru Music Edition, and dupeGuru Pictures Edition
We also recommended dupeGuru for finding duplicate files on Windows. This application is both open-source and cross-platform. It's simple to use -- open the application, add one or more folders to scan, and click Scan. You'll see a list of duplicate files, and you can select them and easily move them to the Trash or another folder. You can also preview them, verifying that they actually are duplicates before tossing them away.
dupeGuru is available in three different flavors -- a standard edition, an edition designed for finding duplicate music files, and an edition designed for finding duplicate pictures. These tools won't just find exact duplicates, but should find the same songs encoded at different bitrates and the same picture resized, rotated, or edited.
This application is utilitarian, but it does its job well. You don't get the shiny interface that you do with the paid Mac apps, but it's a good free tool for finding and clearing duplicate files. If you want a free application for finding and removing duplicate files on a Mac, this is the one to use.
iTunes
iTunes has a built-in feature that can find duplicate music and video files in your iTunes library. It won't help with other types of files or media files not in iTunes, but it can be a quick way to free up some space if you have a big media library with duplicate files.
To use this feature, open iTunes, click the View menu, and select Show Duplicate Items. You can also hold the Option key on your keyboard and then click the Show Exact Duplicate Items link. This will only show duplicates with the same exact name, artist, and album.
After you click this, iTunes will show you a sorted list of duplicates next to each other. You can go through the list and delete any duplicates from your computer if they actually are duplicates you want to delete. When you're done, click View > Show All Items to get back to the default list of media.
That's it? Yup, that's it. We didn't want to recommend potentially confusing Terminal commands that output a list of duplicates to a text file, awkward methods that involve scrolling through a list of all the files on your Mac in the Finder, or applications that require disabling the Mac's Gatekeeper feature to run untrusted binaries. The tools above will do the job, whether you want a barebones-and-free utility or a polished-but-paid application.