When talking of similar answers, I think it should be considered how the answer is perceived from the others. For example, there is a difference between the first group of example, and the second one.
- I think that commenter is a used word.
- I believe that an island is a piece of land surrounded by water.
- I have a feeling that a sybil is a woman able to foretell the future.
- The word commenter is reported by the New Oxford American Dictionary, which doesn't report its meaning. Looking for the word in the Corpus of Contemporary American English […].
- As reported from the NOAD island means a piece of land surrounded by water.
- Looking for the word sybil in the New Oxford American Dictionary, I found the following definition: a woman able to foretell the future.
Answers that refer to a dictionary cannot be considered the copy of an answer that doesn't refer to any dictionary, or that refers to another dictionary (especially if one is the American Dictionary, and the other is the British dictionary). They would be a copy if you would cite a dictionary, but then you would not write what that dictionary says, and copy the answer given from the other user.
When considering if two questions are similar, I would also include the tone of the answer: An answer should not express an opinion about who asked the question; the topic is the question, not who asked it.
Replies that include phrases like you didn't hear well, or your dictionary has been printed when Napoleon was only three years old, make the answer full of denigration. Between an answer that express an opinion about who asked the question and one that doesn't do that, I would surely choose the second one.
Is it an acceptable practice to downvote other answers that are too similar to previously given answers?
I would say it's not acceptable if the down-vote is made from another user who already answered to the question, and down-votes the other answers given to the same question just to put his/her own answer in evidence, and avoid another answer is accepted from who asked the question.
I would then not down-vote a question that is equivalent to another one, and let who asked the question decide which one is more clear for him. Between an answer that is not clear for who asked the question, and one that is clear to who asked the question, the user will probably select the one that is clearer to him; if you down-vote a question which is not wrong, the OP would probably select the other question, even if he understand better the other one (especially if the user understand the down-vote as the answer is not correct).