How to find duplicate files on a Mac (and why some free nothing)

Duplicate finders are old software, and most of them predate the file system your Mac uses. That matters more than it sounds.

A clone is not a copy

APFS, the file system on every modern Mac, can make a second name for the same blocks on disk. Duplicate a file in Finder and you often get exactly that: two files, one set of blocks, no extra space used. Delete one and you free nothing at all.

A duplicate finder that compares names, sizes and hashes will happily list those two files and offer you the space back. The space was never taken.

What a careful search looks like

1. Rule out anything with a unique size. If no other file is that size, it cannot be identical.
2. Compare the first and last few kilobytes of what is left. Cheap, and it eliminates most near-misses.
3. Hash the survivors in full.
4. For anything important, compare byte for byte before calling it a duplicate.
5. Ask the file system how much space each file really occupies, and drop the clones.

Choosing which copy to keep

Decide the rule before you look at the list: the newest, the oldest, or the one with the shortest path — which is usually the one in the sensible place rather than three folders deep in a download. Then check the list, because a rule applied to a thousand files will get a few wrong.

Where duplicates actually come from

• Photo libraries imported twice from different devices.
• Downloads folders, where the same file arrives as file.zip, file(1).zip and file(2).zip.
• Project folders copied "just in case" before a big change.
• Music and video libraries merged from an old Mac.

DiskDuo runs exactly the search above, marks APFS clones so they are never counted, and stages the extra copies for review rather than deleting them.

Download DiskDuo