I notice often on my live board (3.0.12) that users are able to submit the same post.
On my local machine I can reproduce this with version 3.1.0-RC3 in debug mode by just hitting quickly the submit button. I'm not able to do this when in non-debug mode, but maybe that's because the response comes too fast for me to trigger it. I also cannot do this on the 3.0.12 live board (maybe at times when responses come slower, it might be possible).
However, in all cases (live 3.0.12 and local 3.1.0-RC1) I can browse back in history and simply re-submit.

How I would probably solve this: with a hidden token in the form that is stored in the posts table and which is checked for before a new insert.