Most webmasters do not realize this, but a lot of the content on lot of websites can be accessed from multiple URLs. A simple example would be where www.example.com and example.com leads to the same page. This is a fatal mistake in Search Engine Optimization and search engines penalize you for duplicate content. The correct configuration would be where the above two urls will lead you to the same page but example.com will redirect you to www.example.com with a 301 (Moved permanently) status which will not result in search engines penalizing the page. It is very easy to configure 301 redirects using Apache .htaccess file and the process is the same for a Drupal installation also.