I've been getting the same problem for the last couple days. I just refresh my page and it shows my post, so I haven't had any double up. But I also get the same thing trying to send a PM. And I don't know if this is happening to anyone else, but when I enter a thread, I have a reply box open with a quoted message that I have already replied to.I got that Oops message when I posted here yesterday. Tried to post again and you can see the double post result above.
Edit - this post went through fine and no Oops.