Suggestion to link cross-posts.
Can cross-posts be linked so that replies end up in one thread, instead of scattered all over 2 or 3 or more?
Just throwing the idea out there. Probably not original, and the idea is not to save disk space, but to lessen the need to read and reply to several threads, possibly missing out on a great reply (or your great reply being missed).
The downside is that different forums might merit different replies, based on the group membership, but I am guessing that would be minor. (And one could always just do as we do now, and post the same title separately in a really focused group)
Today, one can create a thread, and then give its URL in other forums, but as an OP, that's pretty thin.
(old time Unix person, who used links a lot. Still do. ) 
= new reply since forum marked as read
I don't know if your suggestion is workable but it sounds good. 
I am sure that everything's an entry in a database (set up a raft of these php-MySQL and similar apps)
I can see where people only check certain forums, and hence, the cross-posts.
No code change is trivial .... if you want it to still work.
Like a self-parking car!
