I remember a story where someone uploaded a large file known to be popular (a new torrented movie if I recall correctly) that would have a reasonably chance of

I remember a story where someone uploaded a large file known to be popular (a new torrented movie if I recall correctly) that would have a reasonably chance of already being in someone elses dropbox. The file uploaded in an instant, lending a lot of credibility to the theory that the software hashes a file and looks for the same hash in a central database, and if there's a match doesn't bother to upload the file but simply adds a pointer to it.