A question is an exact duplicate of an older one.
A new answer into the new post has a suggestion based on a new feature introduced, this answer does not exist into the old post.
What should be done?
Keep it open? Close it and move the answer to old post? Something else?
Just close the old question as a duplicate of the new one. Age is not really relevant when deciding which should be closed and which should be the dupe target: you just pick the best question, or the one with the best answers and use that one as the dupe target no matter which was posted first. It's absolutely fine to close old questions as duplicates of newer ones.
The idea of dupe closure is to ensure that users find the answer. So whichever Q&A is better, that is the one that should be kept as the dupe target.
