I'm not aware of any site that would permit many people to post the same or very similar answers to a question. I thought that this was rather obvious, but there's no place in the Help Center to point someone to when their answer is down voted and/or deleted because it doesn't add new value over any of the existing answers individually or the sum of the existing answers.
A small header and a couple of sentences on the /help/how-to-answer page would be extremely appropriate. I would insert it between the "Answer the question" and "Provide context for links". I suppose it could also be a part of the "Answer the question" paragraph, as well.
I don't have any suggestions for the exact wording, as long as it clearly communicates that an answerer should read the existing answers before posting a new one and add new or updated information.