If there is a policy that questions are duplicates if there is an existing answer to a different question that answers them, then I think this policy is incorrect.  If there is an answer to a question which answers a different question, then I am inclined to mark that answer down.  Clearly a question and answer site is at its most useful if people can find answers to their questions as they arise.  Insisting that they then work their way through questions that don't answer* the question that they are interested in in order to find bits of the answer to the question they are actually interested in is inefficient and unhelpful.

*This was previously "relate to" rather than "answer" - edited for clarity, but noted to preserve the sense of the comments below.