What is the need to keep questions which were closed because they were duplicates? Why aren't these questions deleted? Do these questions provide some useful information even if the question is a copy of another question?
Because the closed question...
- may have better wording
- may be a frequently-searched term
- may have a better description
- may produce more answers (before it is closed) that can be merged with the original question.
and thus acts like a bookmark to the first question. This way, we have less duplicates, in theory anyway.
Jeff wrote about this exact issue at length in his blog post Dr. Strangedupe: Or, How I Learned to Stop Worrying And Love Duplication:
One thing I want to be clear about, though, is that duplication is not necessarily bad. Quite the contrary — some duplication is desirable. There’s often benefit to having multiple subtle variants of a question around, as people tend to ask and search using completely different words, and the better our coverage, the better odds people can find the answer they’re looking for. And isn’t that, really, the whole point of this exercise?
There are no incentives coming from the Stack Overflow system to do janitorial tasks like deleting questions. It can even be argued that they are made artificially hard and unrewarding through all kinds of restrictions. Naturally this causes people to stop doing these tasks.