For most users that have a problems reviewing answers and checking who is what isin a duplicate posted answer with the same content there is a tool you can use to check the exact timing it took for stuff to happen in a post.
It is called timeline an can be accessed by going to a post's revisions page and replacing revision
with timeline
in the URL, ie:
http://meta.askubuntu.com/posts/3402/timeline
will show you the exact times for creation, modifications, votes and all the rest for this question.
That is probably the best way to check for possible plagiarism in answers and will help you make up your mind in what to do next.
If I spot identical post I normally use this tool to make up my mind, up vote the post I think deserves more and leave the others alone. If there is need to downvote it will happen but not as often as I would like to admit.