Duplicate content is a term used in the field of search engine optimization to describe content that appears on more than one web page, even on different web sites. When multiple pages contain essentially the same content, search engines such as Google prefer to only display one of those pages in their search results.

Based on Google’s definition of Duplicate content:

Duplicate content generally refers to substantive blocks of content within or across domains that either completely match other content or are appreciably similar. Most of the time when we see this, it’s unintentional or at least not malicious in origin: forums that generate both regular and stripped-down mobile-targeted pages, store items shown (and — worse yet — linked) via multiple distinct URLs, and so on. In some cases, content is duplicated across domains in an attempt to manipulate search engine rankings or garner more traffic via popular or long-tail queries. (1)

Now, how you are going to deal with duplicate content? Here is a video-tutorial from SEOMoz CEO; Rand Fishkin; about on how to deal with duplicate content:

SEOmoz Whiteboard Friday – Dealing with Duplicate Content.