I, in the post about the URL Canonicalization Lesson, wrote:for search engines, http://www.mydomain.com and mydomain.com are not the same. In searches, people (especially those who do not know of your site, in fact, those who already know of your site would most probably not be searching but go to your website directly ) do not necessarily type in http://www.mydomain.com or mydomain.com. They usually type in queries for whatever they are searching for. If your keywords fit their query terms, your listing will appear, but only one (either domain.com or http://www.domain.com) will be listed in the search results. That's how search engines are. Because search engines think that the www and non-www addresses are different from each other, they treat one as having duplicate content and will not display it in the listing.
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