I recently came upon these questions on SU and U&L. I left a comment to the author on both posts linking to the meta.SE thread discussing cross posting. Should I also vote to close or flag for moderator attention? If so, under which category should such questions be flagged or closed?
It's usually enough to just say that it's been cross posted. Don't forget to include a link to the other version(s), so we can take a look ourselves.
If you want to be super helpful, tell us whether the question has already received an answer on the other site(s) it's been posted on. In that case we can usually close it or even delete it here.
If it doesn't have an answer on either site, please only flag on one site. We'll deal with contacting other sites' moderators and resolving the issue. Alerting moderators on all sites will probably lead to even more confusion. In the worst case all instances of the question would be closed or deleted.
If you see comments of the sort "Ask on Super User!", flag them for removal, maybe tell the poster that comments like these only encourage cross posting, and ask them to rather vote to close / flag for migration instead. This happens quite a lot for Stack Overflow → Super User migrations that also end up as double posts because the OP re-asked instead of just waiting for migration.