There are different cases you need to judge.
The answer is a verbatim copy – in this case, one answerer went ahead, copy-pasted the contents of another answer into theirs. Character by character. Ctrl-C, Ctrl-V. This obviously happened after the other answer was posted. This is somewhat unfair and I guess you could revert that edit and/or flag for moderator attention.
The answer is not a verbatim copy – it either:
Takes an idea from another answer and presents it in the answerer's own words. This is something I wouldn't flag. First of all, you might not even be able to tell if there was "plagiarism" involved or two people were just thinking the same. How often do you sit over an "edit" window, adding information to your answer without even looking at the others?
Even if someone appended more info to their post, everybody else is free to do the same. The ultimate goal is the best answer, and this is something that may need to evolve over time. You could come up with a better explanation, more details, further links, et cetera.
Points out the obvious. For "easy" programming questions, like "How do I … in Java?", there rarely is more than one type of answer. With the high traffic on SO in general and especially for certain tags, expect very similar answers to be posted within a few minutes. You can't accuse anyone of "stealing" other answers here. I wouldn't flag in such a case either.
After all, remember that posts should be voted on individually, without looking at the others. So unless their content was a 1:1 copy without disclosure or proper citation, you should vote on the merits of the post itself. You can't always accuse those who are answering of stealing ideas from other answers, unless it's really evident. Neither can moderators, and this is why there's rarely a need to flag these kinds of posts.