A question was posted to m.se. A few hours later, the same question was posted to MO. The MO post was migrated to m.se, so m.se then had two identical questions. I tried to vote to close the migrated version as a duplicate of the original version, but couldn't, since the original had no answers.
So I voted to close the migrated version as off-topic, giving the custom reason that it was a duplicate of the original version. Soon, enough people voted to close that the question got closed as off-topic.
Was this a good way to handle the situation? Would it have been better to flag the migrated version for moderator attention?