You may have flagged the question as a duplicate, but you have not voted to close the question because you don't have the reputation to do that yet. When you flag a question as a duplicate, it shows up in the review queues of users who can vote to close so that they may decide what to do with the question.
That the question was not closed the first time indicates that the community did not agree with you. That your flag was marked helpful indicates that the flag was, er, helpful. Either someone agreed with you, or a moderator felt that while the question does not warrant a unilateral closure, it is a bit of a border case and it was helpful to bring attention to it with a flag.
In a comment under an answer to your earlier question on the same topic, Do we accept duplicates of questions that are posted by the same user?, @DoktorMayhem writes: "it isn't closed because despite the titles, they are not duplicates." I think that the point here is that one question asks whether the I chord of a mode should be preceded by any particular chord, while the other question asks for each possibility for all of the modes.
This does seem like a case where two questions that aren't duplicates of each other may be answerable with the same answers. An answer to the more detailed question might also be an answer to the broader question, but an answer to the broader question may not be an answer to the more detailed one. Yet the fact that the answers to two questions could overlap does not necessarily make the questions duplicates.