Before Stack Exchange, I participated in a similar site. That site had a requirement: all answers must be different from each other, and users cannot copy content from anywhere including Internet.
I will not copy anything here either. I always try to write everything by myself. If I use other's words, I'll make it a quotation (I already know how to do this). But my question is: are there any penalties here if anyone does copy his/her answer? Does every answer have to be unique (not copied)?
As M.A.R. says, copying answers (or any content from other places on the Internet) is only allowed if you provide attribution; otherwise, it's plagiarism.
(note the subtle attribution in the above paragraph)
Sometimes, it's necessary to copy (part of) another user's answer if you want to add more detail to it or just 'build' on it, but can't do this without changing the user's intent. Because every answer needs to answer the question on its own (e.g. in case the first answer is deleted), copying the answer can't always be avoided.
If you see an answer which copies another answer (even with attribution), but doesn't add much value otherwise, you can always downvote it: the tooltip on the downvote button says "this answer isn't useful". But there is no inherent penalty on copying other answers.
