This question already has an answer here:
This question was recently marked as a duplicate, but when I reviewed it, I noticed that the question it was supposedly a duplicate of was asked nearly a year later. It seems, however that there would be an automatic limitation that would prevent marking a question as a dup of a future one, but there isn't.
One solution would be to just, vote to leave it open with a comment explaining why, then mark the other as a duplicate of the older one. However, in this case it cannot be done because the older question doesn't have an upvoted or accepted answer. I simply left a comment on the other one linking back, but I have had debate about a similar incident in the past.