I'm going to disagree with Ash, as well as quite possibly others, here.
To me, an older question can be a duplicate of a newer question. It's not a given that the newer question must be the one to be marked as a duplicate of the older one.
Specifically, one situation in which an older question can plausibly be a duplicate of a newer one is when the newer question is significantly better in some reasonably objective manner.
Normally the newer question will be a duplicate of the older question, because the older question existed when the newer question was composed and posted. However, pretty much the purpose of duplicates being marked as such is to:
- serve as signposts, allowing people to find existing answers to very similar or outright identical questions
- reduce duplication of effort in answering
If the newer question is better posed, has better answers, and is better received (for one way to judge that, look at the ratio of question votes to question views), it's not entirely unreasonable to mark the older question as a duplicate of the newer one, rather than the other way around.
While marking as duplicate is intended to help the OP, it's also intended to help the community as well as later visitors. If marking as duplicate in one direction is likely to be significantly more helpful than doing so in the other direction, then go ahead and break with tradition if doing so is likely to be more helpful in the long run.