Merging when double posting

If an user wants to post in a thread and it'd be his second post a row in this thread, it'll merge his posts.

Author: bryn
Submitted: 16th March 2005
If somebody writes a new post in thread X, and he wrote the lastest post in this thread, it merges the new post with the previous one. It also prevents to add a post to post counter. But, however, it marks topic X as a topic with new posts and puts this topic into "Last post" column in the category list in main page.