It seems to me that a question should be marked as duplicate, even if the earlier duplicate question does not have an answer that is accepted as "correct." The issue arises from this recent question. I suggested that it is a duplicate of this question. @FuzzyChef stated that the "duplicate" did not have an accepted answer.
It seems to me that if the criteria for marking a duplicate is that the earlier question has an "accepted" response, we might have a larger number of duplicate questions, perhaps leading to confusion down the line. Wouldn't it be better to mark the second question as duplicate and direct the community to revisit the first (earlier) question? Beyond just marking as duplicate, is there a mechanism for this community redirection?