Not sure if this is an answerable question, to be honest. It would be quite difficult to draw a line here, but I was wondering how flexible should we be about duplicated questions.
Answers can vary depending on time, who was online at the moment the question was asked, or how long did it take for the OP to choose a correct answer (short time = less answers). So even if two of them ask the same thing, maybe the answers are different. Most of us don't add answers to already resolved questions, unless we come across them and happen to remember the source, etc.
More questions means more chances of appearing in the first positions of search results. Should we somehow agree on when to close a question as duplicate? For example, say, when a duplicated question has more than 4 positive votes, or more than 3 answers (that by the way improve our beta stats), leave it open, or something like that. Not set in stone, of course.
Example: Can I take images from Google image search as a base for my creations? vs Derivative work? . It would be a shame to close questions with so many votes, that give the (correct) impression of a site that is alive and evolving.