Oooh! Ooh! I can help here!
Legoburner explained this in an earlier thread I started, "Double Posting Shenanigans"
Legoburner wrote:
The first error here is a display glitch - click refresh after posting to reload the thread and the second post will be gone. It generally happens when you view a thread, make a post and then before your post is added, another person makes a post. This confuses the cache which resets itself, but the first view (your view after posting) will have that double post.
The second error will pop up in the event of an actual double post and is a combination of some other glitches and the first error. Both are known and on my to-fix list.
and
Legoburner wrote:
It will continue to happen until I finally build up enough evidence to find the error. It is an incredibly boring section of the code and does not happen too often so I am quite lazy about fixing it!