Ok, the only explanation I can think up is due to web based hotmail users clicking the send button a few more times than necessary (Some people are addicted to double clicking when a single click will do the job). Double clicking that little send button will send you two emails, + tripple clicking would probably send you tree copies.

This is the only theory I can come up with, not a real technical one sorry. I too have experienced multiple copys of the same email, and this is the only theory I could think up. I haven't tested it or anything. But in the end, who really cares. Read one, delete the other. There are worse things that could happen to you than you receiving the same email twice.