AFAIK, Google mail servers (gmail) can detect duplicate files. For example, if you receive a big file as attachment that exactly exists in someone else's inbox, google avoids storing the file twice, but instead it will make pointers to the same file.
This is a great feature. For instance, if a file is K MB and is stored by U users in their inboxes, it will be stored once, and ( (K-1) * U ) MB will be freed.
I am wondering if there is any open web-server application that supports this feature.