How do I stop double mails from being sent??

When I send out a mass email, or someone sends an event invite to members, It sends TWO emails, which makes NO sense to me. How can I stop this needless waste of mailings? One is being sent with (unknown sender) as the sender, while the other one displays the correct sender. I'm running a virtual dedicated server through Godaddy, and they only allow 1000 mails per day, I need to cut these useless mailings out. Thanks.