Somewhere around 8-8:10pm P_T on 2012/09/10, this functionality went sideways. Lots of double-posts auto-merged with identical content in both halves.
It could people just double-committing replies (or browser bugs) but I saw it in 4 posts in that period by different authors.
Sidenote: During that period, the "New Posts" page showed all 4 posts but I couldn't navigate to any of the posts for 5-10 minutes. I tried on two different computers, both using IE.