This behavior is unconscionable when there is such an obviously trivial technical solution to this problem. Here is my quick and dirty technical solution. (1)

This behavior is unconscionable when there is such an obviously trivial technical solution to this problem. Here is my quick and dirty technical solution. (1) Place a restriction: only allow users who have uploaded a given file to download that file. In essence, keep an "uploaded" flag for each file/user. (2) Challenge-response to validate local copy of a globally-known file: To continue receiving the benefits of de-dup, don't actually upload an already globally-known file, but perform a challenge-response with the client on the contents of the actual file. This will still leverage most of the benefits of de-dup w.r.t bandwidth savings.