When flagging a thread with the big X, we can choose "Rules violation" or "wrong forum."
It would be nice to flag "duplicate," and if feasible, just provide the threadID of the (older) thread we claim this new thread is a duplicate of.
The action taken by the moderator would be
1. Merge (if possible given BGG's database, I don't think I've ever seen a merged thread, so I think this isn't doable)... if not possible, then:
2. Lock and Link (A quick template post:
This thread is locked for being a duplicate of (threadLink).
3. At moderator's option, if there are no posts other than the first post, the moderator can simply delete the post and GM the user.