I had that happen to me a number of times, and I've seen others post multiple times by accident. It always happens when you click on the submit button and nothing happens, so most people's natural reaction is to click the button again.

You just have to know that the first click actually worked. If you have a long post that you don't want to re-write, I suggest copy-pasting it while you check to see if it was posted or not. My posts are usually short, so I just gamble with the risk.