Is there any routine to decide which one of the duplicate questions should prevail? The one which has been in existence the longest? It would probably not be good if both were accidentally shut down because they were each others duplicate.
When faced with many potential duplicate's to point to, point it at the duplicate you think will help the user the most.
This might not be a sink of the duplicate graph: perhaps the 'most popular' duplicate is overly general, and the current user would benefit from an intermediate duplicate's solution.
When faced with two questions only and deciding which is duplicate, it's well established in the links posted by other users that order of occurrence is not an overriding concern. Try to judge which one is asked and answered the best.