This question seems to be a duplicate of this one, but it is answered, and the possible original isn't. In this case, should the answered question be flagged for closure as a duplicate, or should the older question?
Flag the question that has the least amount of valuable information (between the question and answers combined) as a duplicate of the question with the most valuable information, regardless of when they were posted.
If the two questions both have lots of (different) valuable information then flag the post and ask a mod to merge them; they'll decide on their own which should be merged into which.
Note that you also can't close a post as a duplicate unless the canonical question has an answer (unless they were both posted by the same user), so in this particular case there isn't even a choice; close the one without an answer.