The symptoms described are: The user will post, but the page doesn't update or displays the "busy" animation in the top-right for more than 30 seconds. If the user presses "Post Reply" again, nothing happens. When they refresh the page, they've posted twice.
I've taken a quick straw-poll (http://www.nucastle.co.uk/index.php?threads/74142/page-3) and the affected users seem to be running either Google Chrome or Safari.
Any ideas or suggestions appreciated