I think it would be helpful for a user searching on a topic if the duplicates are linked in both directions, so one find’s the maximum of possible answers. What do you think?
I would say that the amount of linking depends on the answer to the question: "If a random searcher came across the original question, how useful would the duplicate be?"
If a lot, then there should be an eye-catching link such as is already in the other direction. If not a lot, then the current linking is probably okay.
(Incidentally, I don't think that merging is always a good way to resolve such matters. It's a rare pair of questions that can be truly merged without losing any information. And having it all in one place isn't always the best idea. Often one wants the most likely scenario right there with all the little extras easily reachable but not in such a way as to confuse someone.)
They are linked bidirectionally already, but the older question doesn't get a box as eye-striking. Example: How do I create an indentation in the first line of every paragraph? is closed. Its duplicate, Paragraph indenting not working, shows it in the right column as a "linked" question:
However, it'll list all questions linking to this one there, not only the duplicates. So every time someone comments "related question: xyz", this list changes. The list "Related" is produced based on tags and title keywords, I think.