When the user pressed the submit button, it should be disabled until the ajax object state changes.
This way accidental double postings can be avoided, as sometimes it looks like nothing happened when you pressed submit.

Also, if it takes longer than a certain amount to fetch the new post, you could maybe just refresh the page.

Just little suggestions

Another suggestion on that same idea: I don't know if there's a mod for this, but somehow checking if two consecutive requests have the same topic and post content and rejecting the second if they do.

or merging them.