|This page in a nutshell: Web sites that contain information that is directly copied from Wikipedia cannot be used to establish notability or verify the accuracy of any information on Wikipedia|
A Wikipedia clone, also called a Wikipedia mirror site, is a web site that uses information derived wholly or in large part from Wikipedia. The information displayed on the site either may come from an older version of one or more Wikipedia articles that the site has never updated, or may be designed to update the information each time the respective Wikipedia article(s) are edited. Some Wikipedia clones are robotically created without human inspection to verify their accuracy. Most run advertisements to generate revenue.
Wikipedia cannot be used as a source for itself, and therefore, information found on such clone sites cannot be considered a reliable source to vouch for any information added to Wikipedia. Clone sites cannot verify whether any statements are correct, nor can they help establish notability under any circumstances. They may not be listed as external links in any articles.
Very often (but not always), information is removed from Wikipedia because it has been found to be inaccurate. Many hoaxes and urban legends have been started on Wikipedia (see Wikipedia:List of hoaxes on Wikipedia), and many continue to be introduced to this day. In some cases, it has taken several years for the discrepancy to be discovered, and by the time the truth was learned, these rumors had spread to dozens of other sites accessible via a web search. In some cases, false information was later reintroduced into Wikipedia, cited to clone sites, by an editor who was unaware of the past and mistook the clone site for a reliable source.
Also, some sources that seem on first glance reliable may have information directly quoted or obviously paraphrased in large part from Wikipedia. Information obviously originating in Wikipedia should not be used to reference an article.