If someone sends a tree mail (or a forum post or any other form) and then clicks on the back button, the browser will ask them if you want to submit the form a second time. If they click on yes then it'll re-send it. This causes mail to be submitted twice.
I detect that on the server and if you try to send the same mail twice inside of a 15 minute timespan it will ignore the second submission. But after 15 minutes it figures that it's a new mail. I used to have it set to check for several hours but I backed it down at some point to 15 minutes. I guess that's not long enough and I'll adjust the number again to see if I can make this not happen again.
