It's happened to me a couple of times recently, obviously I'm not doing it on purpose. I find when I use my iPod touch to make a reply somtimes I click the submit button to make a post then it goes back to the same screen as if I've just written the message. Click submit again then it goes through the normal process but then I see I've made 2 posts.
Only happens when I use the iPod, never the laptop or my phone via Wap.
Maybe with the rise of the iPhone/iTouch it could explain why we see it happening, but for a technical reason - who knows