There seems to be a general feeling (if not policy) to the effect that questions should not be simultaneously posted to MathOverflow and Math StackExchange (Math.SE). (I think there may have been a meta thread from which this was the conclusion, but I cannot find it.) However, many new users are not aware of this and do it anyway. If I notice such a question here, assuming it is actually appropriate for MO, what should I do about it?
Leave a comment with a link to the Math.SE question?
Leave a comment alerting the user that this is considered inappropriate, but not recommending any further action? Say "it's ok for now, but don't do it again"?
Leave a comment suggesting that the user should delete one of the posts?
Vote to close?
Flag for migration?
(Note: Since this is not Meta.Math.SE, this question only concerns what should be done with the MO copy of the question.)