It's good to embrace duplication, that's how people type many things to ultimately land on the same question. We should never discourage contribution, so we (as in Stack Exchange) feel that it's a mistake to discourage answers to known duplicates, and it's even worse to 'penalize' people purely for answering something when 'they should have known better'.
To a Googler, a nice answer to a marked duplicate helps them realize that they're on the right path, and that the link at the top probably leads to more relevant information. So it satisfies the case of serving future reader's interests.
It helps the person that asked the question feel like their time was well spent, and that they were well received after getting a human response to their question before being told "now go to window 111-a and look for your answer there"
It helps people that want to answer find some way of contributing. More answers don't hurt.
Since there's many cases where folks answer duplicates (some unaware of a duplicate existing totally, others aware, but not voting to close, others answering and voting to close) - lets' not busy our brains with sorting out some kind of rule the UI is never going to enforce anyway - just answer questions when you feel like you can and should, and respect that it's a little different for everyone.
What matters is everyone involved finds value in their time here.