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Dropbox had to quit doing this 10 years ago

https://news.ycombinator.com/item?id=2478567#2478608

Apparently Dropbox notices they already have that file, and instead of you uploading it they just make it appear in your account.

The first time that backfires, though…

They apparently do but only within a users computer (or account, I don't remember). I would be pretty uncomfortable they comparing my files checksum to a master list of sorts

This is exactly what Dropbox does: they hash a file, compare with all known hashes, and deduplicate accordingly. The file is then encrypted with the hash of its contents.