Duplicate Content term explained
Denotes a non-unique, already existing (copied) content. Search engines do not want to index duplicate content, for example, user friendly page for printing can be quite unfriendly for search engines.
Some sites are copying your content with others via RSS or manually, or banishing through a special synonymizer. At the moment the algorithms of search engines are quite useful and they can distinguish such content, and to assign the authorship of the original. Websites with duplicate content often fall under the filter non-unique content.