Recently, two identical questions (copy-paste) were asked by the same user here on MSE, as well as on Stack Overflow. The question is on-topic on both sides, and the question on SO is currently getting close votes as a cross-site duplicate (custom close reason).
As known, cross-posting is discouraged. However, there are no guidelines stating what the correct way to handle these are, once they're posted. To make that clear: this question specifically asks about the correct action after it has happened, not whether it's discouraged or not (which, again, has already been established in a different post).
The tag wiki itself currently says:
If you spot a user cross-posting, please consider politely explaining to them in a comment why we dislike cross-posting.
This comment is also referenced:
this can be OK, so long as the question is tailored to each audience on the different sites and is materially different in each case. Just to be 100% clear, copy-pasting a question across sites with no changes is considered abusive behavior. – Jeff Atwood♦ Jan 15 '11
If you read into that literally, that could be seen as a suggestion to use the rude or abusive flag, but that seems overkill, as well as leaving out context for any future readers, as well as OP and moderators who don't know it's a cross-post.
However, in revision 7 of the tag wiki, advice saying to mod-flag was the correct action was removed by a 20k user (not sure if this is supported by meta or not).
The tag wiki also mentions migration, but this isn't valid. In this specific case, the migration would be targeting MSE from SO, where the question already exists.
So far there's comments, flags (outdated?), as well as migrating (not an option in this specific case, as it would be closed as a duplicate of OPs other posts on the site). Migration is also mainly used on sites where it's off-topic, but that doesn't cover what to do when the post is on-topic on all the sites it's posted to. An R/A flag doesn't seem like the right way to go about it either.
There's already this discussion that asks whether close votes are the right way to handle it, which pretty much only states no, but like the FAQ, doesn't mention any alternatives. However, following a discussion here, it was discarded as an unofficial opinion. This is also just one specific type, and as this specific one says close votes isn't the way to go, that doesn't answer the general question: what should be done about these, if anything beyond what's outlined in the current state of the tag wiki (leave a comment)?
TL;DR: When a question is copy-pasted (not tailored, hence falling under what Jeff Atwood refers to as abusive) onto two or more sites, and it's on-topic on both, what is the right way to handle it?