In  a few days back...Google, Yahoo, and Microsoft announced support for a new link element to clean up duplicate urls on sites.that is called canonical URL tag
See http://www.seomoz.org/blog/canonical-url-tag-the-most-important-advancement-in-seo-practices-since-sitemaps for the explanation.
canonical url tag is to help webmasters and site owners eliminate self-created duplicate content in the index ..........
You can simply add this <link> tag to specify preferred version: <link rel="canonical" href="http://www.rathoredesign.com/nid-india-nid-coaching.html" /> inside the <head> section of the duplicate content URLs: http://www.rathoredesign.com/nid-india-nid-coaching http://www.rathoredesign.com/index.php?option=com_content&view=article&id=165 And Google will understand that the duplicates all refer to the canonical URL: http://www.rathoredesign.com/nid-india-nid-coaching.html Additional URL properties, like Page Rank and related signals, are transferred as well. This standard can be adopted by any search engine when crawling and indexing our site.
Generally speaking, adding canonical tags would only reduce the number of indexed pages if there were a large number of duplicate pages already indexed. There's certainly no penalty for using them, even if there were no obvious need for it.
Hi, canonical URL's means when your URl redirect with Non www or www on the same time on servers so this is canonical issue & you should take a help of 301 redirect!
URL normalization (or URL canonicalization) is the process by which URLs are modified and standardized in a consistent manner. The goal of the normalization process is to transform a URL into a normalized or canonical URL so it is possible to determine if two syntactically different URLs may be equivalent.