Duplicate content is a term used in the field of search engine optimization

Search engine optimization

Search engine optimization is the process of improving the visibility of a website or a web page in search engines via the "natural" or un-paid search results...

 to describe content

Content (media and publishing)

In media production and publishing, content is information and experiences that may provide value for an end-user/audience in specific contexts. Content may be delivered via any medium such as the internet, television, and audio CDs, as well as live events such as conferences and stage performances...

 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

Google

Google Inc. is an American multinational public corporation invested in Internet search, cloud computing, and advertising technologies. Google hosts and develops a number of Internet-based services and products, and generates profit primarily from advertising through its AdWords program...

 can penalise/not display that site in any relevant search results.

Types

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.

Malicious duplicate content refers to intentionally generated search spam

Spamdexing

In computing, spamdexing is the deliberate manipulation of search engine indexes...

in an effort to manipulate search results and gain more traffic. Users do not like to see the same content listed multiple times.