Content Management Systems are often faulty or suffer from bad programming which makes them full of errors such as those which allow duplicate or largely similar content to find a place under multiple different URLs. Webmasters are often guilty of using inconsistent link structures all over their website which can also cause the same content to get indexed with different URLs. The most authoritative version of a URL is without a doubt the canonical version which has been formally indexed by not one but multiple major search engines. These search engines have been known to use some accurate measure such as Pagerank to determine whether the version of the URL under consideration is the canonical version.