I wonder why the forum software can't be smart enough to do what NNTP newsreaders have been doing for decades: merge the discussion and track it across multiple sections).
Whew. It's been a while since I messed around on Usenet. Was fun for a while, for sure. IIRC, a newsreader will generate a unique message ID when it posts. Not sure how that happens, but it's also the identical message that gets sent to multiple newsgroups, so things such as message digest can also be used to find duplicates. A cross-post on a forum might differ in some trivial way, such as inclusion of a comma, or an extra space someplace, unless the poster is really always using the computer's paste buffer to post multiple messages. It might be possible to do some filtering of ancillary characters, such as whitespace and punctuation, and hash the result, to find duplicates. My experience is that most forums rely on users doing moderator notifications, and then the moderators merge the threads.
Obviously, across multiple forums, this is problematic, though with all the right bits in place, I can squint and envision the possibility of some sort of XML-RPC sort of thing which could be used among cooperating forums. Not sure what the incentive is, except to find more answers than were offered in places you're frequenting.