I don't think there is much you can do beyond what you have discussed and what is already in place. You just have to trust the users looking at the question to be responsible to look for duplicates first (this one is hard to trust at times), give good answers, vote appropriately, and go through the close vote queues to agree on duplicate votes already cast. People are flawed, so this is not always ideal (as you have shown), but it is what we have.

Something that you did not mention that helps with HNQs garnering many answers is protecting questions (which has been done for the surface area question). This does not prevent others who are trusted on other SE sites from answering with their automatically awarded 100 reputation points, but I think it helps.

Additionally, if you have found better answers to questions that are duplicates you could explicitly mention these in a comment on the new question. That at least brings further attention to them.

I don't think there is much you can do beyond what you have discussed and what is already in place. You just have to trust the users looking at the question to be responsible to look for duplicates first (this one is hard to trust at times), give good answers, vote appropriately, and go through the close vote queues to agree on duplicate votes already cast. People are flawed, so this is not always ideal (as you have shown), but it is what we have.

Something that you did not mention that helps with HNQs garnering many answers is protecting questions (which has been done for the surface area question). This does not prevent many or bad answers from being posted still, but I think it helps.

Additionally, if you have found better answers to questions that are duplicates you could explicitly mention these in a comment on the new question. That at least brings further attention to them.

I don't think there is much you can do beyond what you have discussed and what is already in place. You just have to trust the users looking at the question to be responsible to look for duplicates first, give good answers, vote appropriately, and go through the close vote queues to agree on duplicate votes already cast. People are flawed, so this is not always ideal (as you have shown), but it is what we have.

Something that you did not mention that helps with HNQs garnering many answers is protecting questions (which has been done for the surface area question). This does not prevent others who are trusted on other SE sites from answering with their automatically awarded 100 reputation points, but I think it helps.

Additionally, if you have found better answers to questions that are duplicates you could explicitly mention these in a comment on the new question. That at least brings further attention to them.

I don't think there is much you can do beyond what you have discussed and what is already in place. You just have to trust the users looking at the question to be responsible to look for duplicates first (this one is hard to trust at times), give good answers, vote appropriately, and go through the close vote queues to agree on duplicate votes already cast. People are flawed, so this is not always ideal (as you have shown), but it is what we have.

Something that you did not mention that helps with HNQs garnering many answers is protecting questions (which has been done for the surface area question). This does not prevent others who are trusted on other SE sites from answering with their automatically awarded 100 reputation points, but I think it helps.

Additionally, if you have found better answers to questions that are duplicates you could explicitly mention these in a comment on the new question. That at least brings further attention to them.