Many content management systems are configured with errors which cause duplicate or exceptionally similar content to get indexed under multiple URLs. Many webmasters use inconsistent link structures throughout their site that cause the exact same content to get indexed under multiple URLs. The canonical version of any URL is the single most authoritative version indexed by major search engines. Search engines typically use PageRank or a similar measure to determine which version of a URL is the canonical URL.
Webmasters should use consistent linking structures throughout their sites to ensure that they funnel the maximum amount of PageRank at the URLs they want indexed. When linking to the root level of a site or a folder index it is best to end the link location at a / instead of placing the index.html or default.asp filename in the URL.
Examples of URLs which may contain the same information in spite of being at different web addresses:
Catalog (see Index)