John Siracusa has a new Mac app out today that can scan your drive and reclaim space taken up by duplicate files. It works atop some clever features of APFS and has an excellent icon. In the app’s documentation, he writes:
Hyperspace searches for files with identical contents within one or more folders. If it finds any, it can then reclaim the disk space taken by all but one of the identical files— without removing any of the files!
You can learn more about how this is done, if you’re interested, but the short version is that Hyperspace uses a standard feature of the macOS file system: space-saving clones. The Finder does the same thing when you duplicate a file.
I just ran it against the SSD in my MacBook Pro and it reclaimed nearly a gigabyte of space.