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Stack Overflow isn't a tutor site. It's a wiki. Its usefulness would plummet if duplicate questions are allowed, since that would scatter all the answers.
Then it should allow to connect duplicates as sub questions to main question which they keep as original, Wikipedia allow additions to articles after all, i mean if you comment your question under main question, who gonna look at that?
People do look at that eventually, but not as fast as some people want.
It's also weird to me that people seem to primarily use it to ask questions (and get butthurt about getting duplicates). It's really rare that I ever don't find an answer there (which often is buried in responses, but still). Like I've virtually never been motivated to post there.