The same have happened to me, with a post. It sometimes happen I guess, maybe because you push the button again to make the thread load faster or something. Annoying as hell.
Lol it's because Something in FireFox or internet is buggig its a problem with me too and after that im angry soooo...You need to click one time at Submit or create and you wait or it will duplicate..
I saw that earlier, I am pretty sure I already took care of it. But don't worry about it. Unless it happens often, and you are doing it on purpose its not a problem. If it happens again in the future, just notify a moderator or myself and we can delete the duplicate.