Introduction to Search Engine Marketing

Keyword Spamming	Stuffing, stacking or overusing key terms in an attempt to appear "relevant" for terms/phrases may negatively affect rankings.
Duplicate Content	Prevalent on the web as a source of lower rankings, duplicating a large portion of a web site or page's content onto another page/site can be harmful to one or both sources. Whether this duplication is in violation of intent or copyright has little to do with how search engines evaluate and list duplicate content.
Uniqueness of Document Text	A document's unique elements are what is generally looked at by the search engines and if the unique elements of a document (the body text or content) is an exact copy of another document (whether that document is on your site or on another), that page's value will often be deeply discounted or even removed from the listings.