adaliabooks: Nice, how did you manage that?
HypersomniacLive: Could you explain how exactly the prevention measure for Quick Post works?
Well, when you press the submit button or Ctrl+Enter the script sends an asynchronous request to GOG's server which adds your post to the topic, and when the request is completed successfully it loads the last page of the topic so that you can see what you just wrote.
However, it was possible to submit multiple times before that asynchronous request has finished, resulting in double-posts. All I needed to do was not allow any more posts to be submitted until the previous request has finished.
adaliabooks: And is it possible to do something similar for non quick posts?
If non-quick posts have the same problem then yes, I should be able to fix that too. Tampering with the website's behaviour is a bit more work than simply updating my own code though.