This question received several answers before it was closed. This behavior seems common, not just for obvious duplicates but also for other bad questions. For instance, I would expect to see a few enthusiastic responses to a question like "what is your favorite function on the integers?" before the close vote went through.
I believe that if low-reputation users were allowed to see the close vote on a question, they would be more reluctant to provide answers to it. With the current system, you have to post a comment like "please don't answer this question; it's a duplicate" with questionable success. But this commenting seems both unnecessary and suboptimal. If people could see the close vote, they'd realize that those trying to close were serious and wouldn't bother answering.
Why aren't users with under 3000 rep allowed the courtesy of seeing that a question is being closed? Ideally people would check for duplicates before answering, but not everyone does and the problem isn't going away.