This is only likely to happen on really slow internet connections. To replicate, enable debug mode and in Debug Options set "Slow POST Responses" to a high number (like 5). You will need two separate windows open with different users logged in viewing the same thread. User A opens the thread. User B opens the same thread and creates a post in it. User A starts typing a post and clicks "save draft" (or allows it to autosave). User A is alerted that a post has been made. User A finishes their post and clicks "Post Reply". User A then clicks the link to see the posts made since they opened the thread. After a few seconds, both posts by User A and User B appear twice! I think the solution to this is probably to remove the "Messages have been posted since you loaded this page." message as soon as "Post Reply" is clicked, rather than waiting for the AJAX response.