I disagree. Answering questions is not a race :-)

I think genuine SoPW answers should only be downvoted if they contain harmful advice - and then I usually prefer to reply with an explanation why it's bad advice.

There is nothing wrong with a good answer that has already been given. The wording of a reply can help clarify an already given anwer, and having multiple "identical" answers can indicate a consensus on how to solve the particular problem. I think it's a waste to downvote perfectly good answers - even if they add nothing to a thread. If anything, it's discouraging to the poster.