Currently, we close questions as duplicates if either the question has been asked before, OR the answer to the new question has been addressed before.
The text when closing a question as a duplicate is:
This question has been asked before and has an answer.
The text displayed when a question has been marked as a duplicate is:
This question already has an answer here:
But these aren't necessarily true. The question may have been asked before, but never received an answer. (Note: I haven't checked the data explorer, so this may be an edge case, but it's still a possibility for a cause of confusion.)