This usually happens when a user has signed up to a contest in the past (either your's or someone else's) and has either forgotten what email address they used or forgot they had signed up. When a person first signs up for a contest using Contest Burner, they create a contest account which is tied to their Facebook, twitter and YouTube user name(s). If they try and sign up using a different email address, we will not recognize them in our system, but we will recognize that their username is in use. The person will have to either use the correct email address, or edit their contest account in order to get this problem solved.