Large search engines attempt to filter their search results by removing any results that duplicate the content of other search results. Such filtering is referred to as "duplicate content penalty".
It is important to understand and identify what "duplicate content" actually is. Duplicate content is generally defined as substantive blocks of text that are copied from one site to another. Some webmasters try to use duplicated content in an attempt to manipulate and influence search engine rankings. The search community still occasionally debates the legitimacy and existence of duplicate content filters, but whether they exist today, or will exist tomorrow, is really irrelevant. Most webmasters have simply accepted the fact that the duplicate content penalty is currently enforced by at least some of the major search engines.