I wrote a post about this a while back (but I can't find it anymore)*
It's understandable & within everybody's right to cross-post questions on several forums. No problems there. It would however be kind, if you were to mention having cross-posted your question on other forums as well, as it's slightly annoying if the person trying to answer the question finds out too late the question has already been answered on one of the other forums. That is why, if a question gets answered on another forum, quickly returning here to inform "us" that the problem has been resolved would be nice too...
This isn't a rule, I see this an extension to existing "netiquette".
I found my original post, it's well hidden (in plain sight) in the rules
even (contradicting this post
), although that speaks only of resolved issues, while simply notifying everyone beforehand about a cross-post seems a better idea.