It's not a massive problem anyway.
Back on topic, I think that even some rules can be enforced easier this way. For example a question: "Is this thread is about a website?" AND IF ANSWERED YES "Have you already made a thread about it?" would reduce on duplicate threads if the poster just didn't know the rules.
I suppose topics aren't posted that often, so to ask a few questions to guide the post to the correct forum wouldn't be too bothersome. The trouble I find is that you can't get a decent mapping between answers and forums without asking lots of questions.
Your proposal to ask questions to tackle the problem of duplicate threads could be useful, however once people are savvy to the mechanism they will simply click no, allowing them to post the topic. Also there may be legitimate cases to make a new topic.
I like the idea of using the questions to tackle other problems, rather than the miscategorisation issue, which is a complex one.
IMO there are several, good intentioned reasons to make dup posts, and I would say a lot of them are posted because of lack of understanding of the rules.