Refers to instances where portions of text are found in 2 different places on the web. When the same content is found on multiple websites, it can cause ranking issues for one or all the websites, as Google does not want to show multiple websites in search results that have the exact same information. This type of duplicate content can occur because of can result from plagiarism, automated content scrapers, or lazy web design. Duplicate content can also be a problem within one website — if multiple versions of a page exists, Google may not understand which version to show in search results, and the pages are competing against each other. This can occur when new versions of pages are added without deleting or forwarding the old version, or through poor URL structures.
Source: HSMAI CHDM Study Guide