Should we leave a comment, copy-paste the answer(s) from CrossValidated, etc.?
Ideally one copy gets closed, but we can only close questions on this end. If it makes more sense on CV, mark it as duplicate. If it makes more sense here, flag it as duplicate on CV.
You can leave a comment pointing to the no-cross-post policy. I would not copy answers, no.
Cross-posting of the same exact question on two Stack Exchange sites is not allowed.
What should you do? Leave a comment on both posts indicating that it was a cross-post and informing the poster about the "no cross-posting" policy. Then, decide which site you think it is best-suited on, and flag the copy on the other site to be closed (flag for moderator attention, with a custom flag, and explain your reasons in the textbox).
Here is some stock language you can use for the comment:
Also posted on Panda Stack Exchange: http://panda.stackexchange.com/q/1.
Please [do not post the same question on multiple sites](https://meta.stackexchange.com/q/64068). Each community should have an honest shot at answering without anybody's time being wasted.
Also posted on Panda Stack Exchange: http://panda.stackexchange.com/q/1. Please do not post the same question on multiple sites. Each community should have an honest shot at answering without anybody's time being wasted.
If it's two different people asking two similar questions, this doesn't apply. You can write an answer summarizing the main idea and linking to the other site, if you want, or just leave a comment with a link to the other question on the other site.