As such there is nothing wrong with flagging older questions as dupes of newer ones if there are no good answers.
But in this case I see several problems with this user's action:
Not only did the older thread have an excellent answer already but the user who marked the old thread as dupe of the new thread apparently knew of the older thread and yet posted an answer to the new thread instead of marking it as duplicate.
The other problem is that I think marking as duplicate should be a peer reviewed process. In this case, one single user appears to have the power to mark as dupe.
The same user has now cast a final vote to reopen the 2014 question. By final I mean he again used his powers instead of waiting for a 5th person to agree that it should be reopened.