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?

  • Something else?

(Note: Since this is not Meta.Math.SE, this question only concerns what should be done with the MO copy of the question.)

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    $\begingroup$ In this context, I think first of all it would be good to alert a user before cross-posting that immediate cross-posts are frowned upon here. Such warning could be displayed whenever one prepares to ask a question on MO whose text is very similar to a recent question on MSE. $\endgroup$
    – Stefan Kohl Mod
    Commented Jan 17, 2015 at 21:53
  • 2
    $\begingroup$ @StefanKohl, is it possible to make such a check between SE sites? If it is, that'd be brilliant. $\endgroup$ Commented Jan 17, 2015 at 22:31
  • 2
    $\begingroup$ I think you shouldn't tell a new user that they did something inappropriate but not recommend any action. That will only confuse them. Maybe you could ask which site they consider more appropriate (explain the difference if necessary) and ask to delete the one of the posts. It is also a good idea to add links between sibling questions. $\endgroup$ Commented Jan 17, 2015 at 22:35

1 Answer 1


A key-thing to do is point out the other post, also for the benefit of others. The main annoyance of the cross-posts is the risk of duplication of efforts. This can be minimized when there is a note with a link. I think one should essentially always do this.

Everything else is less important and can vary on situation. If it is a cross-post I am a bit more likely to vote to close, but I do not always vote to close.

In the former case I might write something like (though I am a bit briefer sometimes):

The question was cross-posted on math.se (giving a link). I believe it is better suited for that site, and vote to close here. Please, rather try to avoid cross-posting or at least provide a link when you do cross-post.

In the later case I might write something like:

The question was cross-posted on math.se (giving a link). Yet, it seems like a good fit for our site. Still, for the future please rather try to avoid cross-posting or at least provide a link when you do cross-post.

In my observation it can be important to state explicitly that one does not want it closed when this is the case; only pointing out the cross-post seems often to be taken as invitation to close by observers.

Normally, in case I close, I simply close as off-topic (generic or the one mentioning math.se) and do not vote to migrate the cross-post. In theory this would typically be the correct option (first migrate, then dupe-close); in practice, though it feels a bit fuzzy, especially since on a site as busy as math.se the necessary dupe-closure could fall through the cracks.

  • 1
    $\begingroup$ If you have an account on Math.SE then you can follow the migration and flag for a moderator to merge the copies. $\endgroup$
    – Asaf Karagila Mod
    Commented Jan 18, 2015 at 6:20
  • $\begingroup$ Good point. Only, my vote might not trigger closure and migration, so I would need to track the situation or trust somebody else will do it. Either way, it feels like busy-work for little gain to me. $\endgroup$
    – user9072
    Commented Jan 18, 2015 at 15:28
  • 9
    $\begingroup$ The top point is really the key to me. Both posts should indicate the crossposting clearly, prominently, and immediately, regardless of eventual action on them. $\endgroup$ Commented Jan 18, 2015 at 19:56
  • $\begingroup$ @Asaf: now that you implanted that idea in my head I actually start doing this more. Thanks so much for that extra work! ;-) $\endgroup$
    – user9072
    Commented Jan 19, 2015 at 20:05
  • $\begingroup$ At least I didn't hack your dreams, four levels in, and made you believe that it's your idea by faking a dream in which your dying father tells you that he wants you to sell a bank and become your own self made man. $\endgroup$
    – Asaf Karagila Mod
    Commented Jan 19, 2015 at 20:10

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