Either you have hit "Refresh" on the sending page or your host could have a problem. Some hosts will automatically "Refresh" the page for you.. What you need to ask your host is to make sure that the option "KeepAlive" is off in Apache's httpd.conf
I have now asked our host to keep alive and that has been done and I don't hit the refresh button either. Yesterdays email to 1500 all got the email twice too. It only happens on one database of customers that I imported rather than our newsletter subscribers. ie it is sending out a Send Email to 1500 rather than the daily drip of the newsletter sign ups, so it often takes ages and then come up with the "you have email in the queue do you want to resume or cancel.