I wish people spent a little more time thinking this through. Lets say you copy the thread: now do we add another reference to that thread from the original poster or will it just be an orphan now. You should rename the thread prefixing it with "duplicate" or it's going to be difficult to determine the master thread. Now you have two threads with the same content. Are you going to synchronize them continuously or are they now just two separate threads with almost the same content, because if the threads are currently open more content could be added. The only thing I can see is to duplicate the "CLOSED" thread and call it an archive with the prefix 'Duplicate'.
Oh I get it... It would be nice to have a backup copy. I went though the same thinking until I really started thinking about it...
The way I handle it in the large board I oversee: The board rules state that all content belongs to the board. You can soft delete the post under your username, but if we find it valuable, then we will re-post it as an anonymous user to the same thread, same post number -- another words we just change the post's owner.
@Marco in post #9 had it right: Do a redirect and call it a day.
If someone really has a solution to all this and has really thought it through, you can contact me and I would entertain writing a mod for it. At the current time I cannot find any logic for doing it. Anyway, I hope this helps the debate in some small bit. I wrote this at 4:00 am on a plane, I hope it makes sense.