Every time I send a message on Thunderbird I get an "extra" copy of that message posted in my "Sent" folder. In order to keep the number of messages down that take up overall data space I then have to erase one of those two for every one that I've sent. I've never experienced this before in any email system I've ever had. How can I shut that duplication off so it only stores the one copy of the message I just sent?