We frequently have questions that are crossposted, e.g. http://mathoverflow.net/questions/137414/systems-of-parameters-for-algebra-a#question.

The only options for now are to close as off-topic, leaving a comment pointing to the math.SE question, or to migrate and leave math.SE to clean up the mess. Neither is that great, and the perfect solution would be to close as a duplicate of the math.SE question. However, if you paste the math.SE URL into the "close as duplicate" dialog, the software responds with "The duplicate question must exist on MathOverflow"

Can we fix this, and allow closing as a duplicate of a math.SE question?