When an answer says something that is already said from another answer, it could be down-voted as not useful. It just repeats what said from somebody else, so it is not adding anything useful; if we were to do the same with every question, we would get the same answer repeated X times each time.
Keep in mind that there are cases where two answers could say the same thing because a user starts to write an answer without first refreshing the page and not noticing there is already another answer saying the same; it could also happen two users starts to write an answer at the same time, but one of those is distracted and hits "Post Your Answer" later than the other one.
In the specific case, the user also added something else the user who answered first didn't say. Probably it doesn't make any difference, since the core of the answer is still the same.
As for flagging, don't use it in cases like this. If there were a user who keeps copying-and-pasting answers given from others, by any meaning, flag those answers. Moderators cannot do anything in cases like the one reported in the OP; users can down-vote the answer that repeats what already said, and for that you don't need any moderator.