If I go to upload file A.zip to Dropbox, and it already exists in someone else's Dropbox (has a matching hash), does Dropbox bother re-upload the entire thing?

If I go to upload file A.zip to Dropbox, and it already exists in someone else's Dropbox (has a matching hash), does Dropbox bother re-upload the entire thing? If not, then perhaps there's a way to test if the files came from Dropbox or not.