Duplicate content is a term used in the field of search engine optimization to describe content that appears on more than one web page, within the same web site. When multiple pages within a web site contain essentially the same content, search engines such as Google can penalise/not display that site in any relevant search results.
Non-malicious duplicate content may include multiple views of the same page, such as normal HTML and a version for mobile devices, printer-only versions of a page, or store items that can be shown via multiple distinct URLs.[1]
Malicious duplicate content refers to intentionally generated search spam in an effort to manipulate search results and gain more traffic. Users do not like to see the same content listed multiple times.