If a question is cross-posted here and on the Math Stack Exchange, what's the policy to deal with these? For example take [this question](http://mathoverflow.net/questions/136162/how-many-distinct-pairwise-difference-multisets-exists?noredirect=1) and [its MSE twin](http://math.stackexchange.com/questions/439894/how-many-distinct-pairwise-difference-multisets-exists).

1. Should one of them be closed?

1. If so, which one?

1. Does the choice depend on where there have been answers?

1. Should authors of answers copy (or perhaps even copy-then-delete) these?

1. If one should be closed, what close reason do you consider appropriate?

 The latter might depend on which of them you close, since I can come up with an easy off-topic reason for closing on MO, but can't think of anything which would be on topic for MO but off topic for MSE, except perhaps for some work-life questions.