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Technically, http://www.example.com and http://example.com are two different sites. So, to prevent the SE's from thinking that you're running two sites with duplicate content (which can lead to one or both not performing well in the SERP's, backlinks split by coming into two different sites, and lesser problems) you need to permanently redirect all of the pages on one version of the site, to all the pages on the other version of the site, i.e., all non-www pages to their www counterparts, or the other way 'round.
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