Is it really necessary to close a question here if it was previously asked on StackOverflow? I would have thought the time to do that would be when /if the question on StackOverflow is migrated. That way we can merge the two.
Especially early on in the life of this site, I would not want to dishearten new users by closing questions that are unique to this site but were asked before on a site they aren't even a member of. Should users be expected to check StackOverflow for existing duplicates before asking their question?