I don't think Copyscape's right for OP's needs - you only want it to only check uniqueness between the articles, right?
In which case, Ambitious12 and Mister Don are right - SpinnerChiefIII will compare articles against each other and spit out a table comparing each article against each other article and % uniqueness. (Copyscape's strength is comparing an article against the whole web, looking for plagiarized content.)
Edit: Coming to think of it, SC limits it to 10 articles at a time, doesn't it?