The same goes for double topic posts, the new comers are quickly advised of the problem and I see no need to penalise them by locking their threads.
I do see the need to remove/lock/archive multiple posts. If they are both/all allowed to remain, it can make the subject very difficult to follow. If an OPs problem is a particularly difficult one with many possible responses, keeping track of things can be a nightmare for all concerned. Because of that, mistakes can be made and the wrong advice offered by a "helper" or the right advice misunderstood by a "requestee" because of the confusion.
It's not a case of penalising newbies; it's a case of helping them to help us to help them if you see what I mean.
For similar reasons, thread hijacking should be frowned upon as it can become very disruptive and confusing for everyone concerned.
I still mod on a couple of forums and we have the ability to split, join and start fresh threads etc without losing any content. I don't know if that's possible with the YABB implementation or perhaps more importantly, if mods and admins want that responsibility/extra workload.
It may necessitate the addition of a "patching" or "temp" forum with mod/admin only accessibility if desired. Such a forum negates the possibility of accidents happening and material being lost.
If any "hidden forums" are formed for these sort of purposes, I know from bitter experience that it's very wise to let it be publicly known that they exist from time to time!
All of this makes for a tidier forum with easier to follow threads for everyone.
As long as there is good reason for intervention and that reason is given in the thread by the mod/admin concerned, I see no problem whatsoever with it.