I can't answer for every moderator, and the closest thing I found to an official policy is this question on meta.SE, but I can tell you my perspective.
In the case you describe, where a user has written a new answer that isn't the exact same text as an old one but has the same information, typically I'll leave a comment telling the new user that their ...
The project is drawing to a close, and I am planning to take down the annotation interface this weekend.
Thank you very much to everyone who helped out, and thank you for all the valuable feedback and suggestions. Hopefully the collected annotations will lead to the development of better duplicate question detection systems, especially for StackExchange.
I've closed the other question as duplicate. With the only answer being link-only, I see no reason to merge it. Due to the dupe-closure, the two posts are linked together anyway, so it can be looked up easily. So in cases like this, you can vote-to-close if your reputation allows, or flag it otherwise.
Thanks for reporting!
What is a duplicate answer?
A duplicate answer is an answer that does not add any information not alerady in an older answer.
What should I do about that?
Be sure to be on the duplicate answer, not the original one.
Leave a comment kindly saying that the answer doesn't add anything new to the question, and thus should be appropriately edited or deleted.