If a person makes 2 posts in the same thread without anyone else making a post in between them (and maybe in a certain timeframe) the posts get automatically merged.
Thus if I would make a post right after this one and before anyone makes one as well, instead of seeing 2 posts of me you see 1 with a small indication that it is a merged post. (I believe it was a small gray line and the word merged somewhere that separates the 2 posts)