This question already has an answer here:
Cross posting is posting exactly the same question on multiple Stack Exchange sites. This is both a common and a highly problematic practice; splintering answers across the network doesn't help people looking for them, which is all our sites are supposed to be about. Very new users, who've yet to learn our strange ways, are the usual culprits of this. I don't think we can blame them, this is (more or less) acceptable behaviour on every other forum on the internet. Furthermore, I don't think expecting them to know they can flag for moderation attention and ask for the question to be migrated is reasonable.
That said, and while cross posting is strongly discouraged, it's not forbidden. There are often significant overlaps between sites, and a question may very will be on topic on more than one sites. Cases like this aren't always handled consistently. There are some that believe only one copy of the question should exist, and then there are others that don't find the fact that an on-topic question has been posted elsewhere reason enough to close it.
I'm in the first camp when it comes to questions that have been copy pasted verbatim on multiple sites, and on the second for when the poster has gone through the trouble of tailoring each version to the audience of each site. The first category is, I believe, trivial to detect and it's the one I'd like to see automatically rejected by the system. Even in the rare occasion when a copy pasted question happens to be on topic on both sites, copy pasting your question across the network is perceived as rude and abusive behaviour from veterans. And more often than not, cross posts tend to attract an onslaught of downvotes and/or negatively charged comments, regardless of their topicality. We are doing a very poor job educating our new users about the faults of cross posting, but a fantastic job at chastising them for it.
If outright blocking is out of the question, then I think at least an automated warning is in order. Something along the lines of:
It seems you've already posted this on Chemistry. Also posting it on Arqade may not be such a great idea, questions are very rarely on topic on more than one sites. Please check out Arqade's help center to find more details on what questions this site welcomes.