[kh99]

This site does something similar, but I don't think there's a mod for it and I don't know exactly how it was done. But I took a quick look at function build_new_post() in includes/functions_newpost.php. I think you could modify that code to check the last visible post for the thread, and if it's by the same user and within a certain time period, modify that post instead of adding a new post. There's already code to check for posting a duplicate so I imagine you could modify that code instead of adding an extra query.