I have noticed that when submitting an AJAX reply to a PM sometimes the AJAX-loaded recent posts will load a post that is already on the page. And of course refreshing the page fixes it. Is that what you are talking about?
I have never bothered trying to track down this problem.
I'm not able to reproduce this and I don't see a bug in the code jumping out.
Stewart could only reproduce it in FF and not Chrome. I'm not able to produce it in either. Can anyone else reproduce this with any consistency? If you can do it here, the most useful thing would be to know the ID (or URL) of the conversation and for the value of this hidden input in the source before you replied: