Recently, I read in Matt Cutts' blog (Google Engineer), he is telling about cannonicalization of URL, which means the basic URL that should consider for google listing and ranking.
Normally, web developers uses several methods to link to the home page by www vs non-www, redirection, duplicate URLs, 302 'hijacking' etc. In most cases we can see links that are similar to the following, but going to the same page:
All the above links are different in technically and web server returns different contents for all those URLs. Here are some tips to solve this issue that Matt Cutts answered:
1) Do not place different versions of links throughout in your website.
2) If Google listed different versions of URLs, try to setting up 301 permanent redirection in web server.
3) Do not use the URL removal tool to get rid of these kinds of URLs.