Kempt
In short
macOS has no built-in duplicate finder. System Settings › General › Storage lists large files, and Finder can sort by size and name. Kempt’s Clean Up finds old installers, exact duplicates and similar re-saved copies in 0.86 s, never pre-ticks a similar copy, and only ever moves things to the Trash, with Undo.
Without any app
See what’s big
System Settings › General › Storage › Documents shows large files and downloads.
Sort to spot copies
In Finder, list view, sort by Name then by Size: “file.pdf” and “file (1).pdf” of the same size are likely copies.
Check before you trash
Use Quick Look to compare both. Keep the one with the clearer name.
With Kempt
Three kinds, separated
Installers you no longer need, exact duplicates, and similar copies (re-saved or re-downloaded versions).
Careful by default
Similar copies are never pre-ticked, because “similar” is not “the same”.
Trash only, with Undo
Kempt never deletes a file. Everything goes to the Trash and can be put back.
Questions people ask
Does macOS have a duplicate file finder?
No. Photos can merge duplicate photos, but there’s no built-in tool for documents and downloads.
Is it safe to delete duplicate files?
Exact duplicates, yes, once you keep one copy. Similar files may differ; check them first. Kempt only uses the Trash.
How fast is Kempt’s Clean Up?
A full scan took 0.86 s and 83 MB of memory in testing.