How many photos do you have twice?

This tool reads each file and boils it down to a fingerprint — a short code computed from the file's exact contents (the technical name is SHA-256, but nobody needs to remember that). Two files with the same fingerprint are perfect copies of each other, byte for byte. Two files that differ by even a single pixel get completely different fingerprints.

That makes this test wonderfully strict. When it says two files are duplicates, they truly are — same picture, same quality, same everything — so keeping just one loses you nothing. It will never flag two photos that are merely similar.

It also means this finds exact copies only. The same photo saved at a smaller size, lightly edited, screenshotted, or squeezed through a messaging app has different bytes, so it slips right past. Those near-duplicates are usually the bigger mess, and catching them takes smarter fingerprints that look at the picture itself, not just the bytes.

And a browser can only check one folder at a time. The real pile-up usually spans your phone, an old laptop, a couple of hard drives, and a cloud or two — the same vacation living in five places at once.