I’m thinking of using WordPress to put up the transcript of a historical trial. John Smith under Witnesses would refer only to his appearances as a witness, whereas under People it would refer to any where he is mentioned. I distinctly remember reading on Blogspot that categories had to be unique no matter where they were in the hierarchy, clicking on John Smith would return all posts in the category, both Witnesses and People. It seemed nonsensical to me: no Pies -> Cherry, Fruit -> Cherry, but it said the hierarchy was only for organization.