What I don't understand (coming from communities where duplicate posts are simply killed) is why this manner of handling is favored.  I do understand that the community cycles in more people who haven't seen all the older links, and so it's nice to present them again for their sake every once in a while.  However, if that's the goal, why start with a fresh discussion thread every time?  Why not just put the old one on the front page again, together with the discussion attached to it?  Presumably the comments haven't rotted.