I have come across several questions which have been cross-posted to MO with no clear indication, such as this question cross-posted here, or this question cross-posted here (note the second example has been edited to disclose the cross-posting, but this was not the case right away as can be seen from the comments).

Of course, if I am unaware that such a question has been cross-posted, the best I can do is ask in the comments. However, if I know that a questions has indeed been cross-posted, should I

• Edit the original question to include a link to the cross post?
• Post a comment with the link?
• Flag the post as a duplicate to keep only the MO version?

To me, it seems that the first two options make most sense. What is the best practice in this situation?