My suggestion, reword this to read: 'This question has been asked before and already has an answer. If those answers do not fully address your question, please modify the question in a way that will highlight the differences which you are experiencing.'

To me this is the single most frustrating aspect of SE. If the other question really did answer the question then can't we trust OP to voluntarily delete the new one? Also, this mostly happens because the OP wasn't asking the question the right way. Great! They found a new way to word an old question. Doesn't that help us with our search results (assuming others are asking the same 'wrong' question)? Also, don't answers ever become stale? Why not keep the new one and move the good answers over? Hint: it's because the new question really isn't the same question; more often it's only similar.