If a gold badge holder for a tag votes to close a question normally, and then later discovers a duplicate of it, can they be allowed to "cast a second close vote" as a duplicate? In the "You have already voted to close this question" dialog, could the "Duplicate" option not be disabled?
For the tags I follow, I tend to read through the questions in reverse chronological order until I start hitting questions I remember already reading. So it can happen that I encounter a question that's missing key details, and I vote to close it; but then find later in the question list that they've actually reasked a question identical to one that got closed. My close vote on the new question isn't wrong, but the dupehammer would probably be more correct and more efficient.