Duplicate content is not always plagiarized content, it refers more to blocks of content that match other blocks of content across the web based on the substantive issues being written about.
Examples of this would be definitive information on the same topic, such as a definition for a legal offense or a scientific definition. Other examples include message boards where the same questions are asked on both sites, such as how to fix a flat tire. All answers that would result from such a question would be substantively similar in nature so as to be considered duplicate content.
Content is sometimes duplicated for the purpose of gaining better search engine rankings, so that when a topic is searched for by a person it will bring up the competitive site as well as the most commonly accessed site for the topic. Duplicate content that is plagiarized will be taken down by moderators when they are found. Bots can be tasked with locating duplicate content.