Duplicate content - definition
Duplicate content refers to blocks of content on separate web pages that match each other exactly or substantively. There are many reasons that duplicate content may occur, both within and across domains.
Legitimate reasons include printer versions of a web page (think brochure on a listing detail page), mobile versions of web pages, and product pages with identical descriptions across many sites (think listing syndication). In legitimate cases within a domain, webmasters are able to indicate the preferred URL in the code.
In other cases, where a preference is not indicated, the engines will make a page choice between what they consider to be duplicated content. Showing multiple pages with the same or nearly the same content would result in a poor user experience.
There are cases where sites attempt to manipulate the search rankings by duplicating content across domains. This excerpt from Google webmaster tools explains their policy:
"Google tries hard to index and show pages with distinct information. In the rare cases in which Google perceives that duplicate content may be shown with intent to manipulate our rankings and deceive our users, we'll also make appropriate adjustments in the indexing and ranking of the sites involved. As a result, the ranking of the site may suffer, or the site might be removed entirely from the Google index, in which case it will no longer appear in search results."