Are duplicates duplicate questions
Are duplicates duplicate answers
The text that is inserted into the question when it is closed as a duplicate gives a clue as to it's intended usage
Something that reinforces this is that you cannot close a question as duplicate if the target has no answers. Duplicates are questions that already have an answer.
When do we close as duplicate
Close as duplicate when the answer to the question is answered by the target - simples.
Allegedly one of the greatest benefits of the SE engine is that questions that already have answers can be closed a duplicates. Sadly it's often way easier to answer a question that you know already has an answer than it is to find the duplicate you know exists.