While I understand your issue, the idea isn't practical. The whole point to duplicates to try to direct users to good answers for the same issue, not delete content simply because someone decided it was a duplicate.
Sure, you would solve the problem of bad answers posted to duplicate questions. But what other "problems" would you be solving:
- Someone posts a decent answer to a question and it is closed as a duplicate 3 years later after amassing 1000+ upvotes. Why should that answer be deleted simply because the question was closed as a duplicate?
- Someone mistakenly closes as a question as a duplicate. Should the answers be deleted?
- An older question (with a good answer) is marked as a duplicate of a newer question because the newer question had a better answer.
- A user answers a question because he doesn't know it is a duplicate (and it is marked as a duplicate at some point in the future). While the answer may not be significantly better than the duplicate, it doesn't suck.
And there a lot of other variations on the same theme. But the answer would still be the same. People answer duplicates. It might because they want to get easy upvotes, it might be because they didn't know it was a duplicate, it might because someone decided it was a duplicate well after the question was answered. Regardless of "why", the short answer is the same.... why should we deprive the community of these useful answers simple because someone thought it was a duplicate.
Rather than automatically deleting good content to prevent the occasional bad answer, maybe you should just deal with the bad answer. The solution for bad answers hasn't changed in 7 years..... downvote crap.
In regards to your other idea about adding additional links, they system already does that in a way. In the "Linked" section of the sidebar, all posts that are linked as duplicates to the question you are looking at will appear there.
While it is not in the question itself, it is a handy way to scan through all of the duplicate and linked questions.
Trying to add them to the question itself is going to be messy. There are 55 linked questions. While not all are doing to be duplicates, a majority probably are. Even if it is just 25 questions that are actually marked as duplicates, how would you design a user interface to hold 25 links without appearing too messy.
The example I used above may not be the norm, it is certainly not unusual. There are plenty of questions with dozens of duplicates, so it will come up more than enough to be a problem.