When people see a duplicate some vote to close, some authors delete it, some don't and some post a link to a duplicate in a comment. It's all very messy for something that is bound continually to happen.

There should be e.g. a This-is-a-Duplicate button, you click it and fill in the URL to the duplicate question. That way you get little "bundles" of questions which are asking pretty much the same thing, and since some questions are not truly 100% duplicates but just near-duplicates, this keeps them together for people who are interested in the general topic.

A question with duplicates which has a list of them right after the question.