A question was marked as duplicate even if it is much older than its "duplicate". The other one should have been marked as duplicate. Can you vote for the question to reopen,please,so that the other one would be marked as duplicate?
When two questions that are duplicates of each other both live long enough to attract answers before getting closed, it's not always as simple as "the older one wins" when marking as duplicates.
It's a pretty subjective process at that point, but we like to try to choose the question with the best content as the canonical one, and close other questions as a duplicate of that one. In this case, the one that remained open has longer and more thorough answers.