If you press submit multiple times, doesn't it throw some sort of "double post?" warning? It may be caused more often by people pressing the back button instead of following the "return to board" link after posting so they don't lose their "new" tags.
I would assume that it's like 95+% posts from phones that do this.
Sometimes on me iPhone, I'll hit submit and then it will sort of sit there and not load. People either get impatient and hit submit again (which isn't necessary), or the phones get stuck in a loop of sorts and send it multiple times.