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Today, I saw this question on Super User. The first comment pointed out that it had been answered on StackOverflow. Would it have been appropriate to flag the SU question as a duplicate of the SO question, or is the dupe flag specifically intra-stack?
Note: I have not so flagged it; I don't have enough rep on SU to be able to know whether it has been so flagged/VTC'd.
It is not possible to close as duplicate for a question on a different SE site.
To demonstrate, here is a screenshot of the error I get when attempting to close as duplicate on that very post:
