I saw this question. 2 minutes later, it was marked as a duplicate. How is this possible?
It was spotted very soon after posting by a riddle expert with a 'dupehammer'; their gold tag badge lets them singlehandedly close a question as a duplicate of another one. You can see this when you hover over the badge:
More information about this feature can be found on Meta Stack Exchange: Increase close vote weight for gold tag badge holders. The idea is that users who have solved so many riddles are likely to recognize when one is reposted.