Duplicate Content: Duplicate content is any text that is identical or very similar to other content.
Search engines do not like to index multiple versions of duplicate content. A duplicate content penalty may be applied by search engines to any sites with text that is too similar. This applies both internally on a single website and across websites. For example, product pages with nearly identical product descriptions might be seen as being duplicated. Duplicating one’s own website multiple times or taking content from other places for use on your website—what would be called plagiarism in most contexts—can also lead to duplicate content penalities.
Search engines algorithms do not distinguish between intentionally plagiarized content and repetition that may be inadvertent, so it is important that web developers constantly strive for diverse content on their sites in order to optimize page rankings. Owners of popular websites may also be affected by people scraping their articles, or reposting them without permission.
Often the appearance of duplicate content is caused by technical misunderstandings, rather than human error. For example, the way a site’s database is built may differ from how the search engine views URLs, leading it to believe the site has duplicated content. Google Webmaster Tools is an easy way to check if a site is getting penalized for duplicate content the developers may not even know is there.