Every community has a slightly different approach. Some communities are quite fast on closing questions, others don't. This is the same for which question gets to be closed as a duplicate of the other. Generally speaking, from the Q/A point-of-view, you want to retain the best question and answer set since that is the one you want people to actually read (I think even Google knows the procedure so non-closed questions will be placed on top in the search results). The other should be closed as a duplicate of that one, regardless of the age of both.
That doesn't mean we want to ask the same question over and over again, just to improve the question. Instead we want to edit the existing question and answer and improve them. If some time later a duplicate is found, you pick which you find the best. If none really stands out, I think it is best to keep the oldest as the main question.