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By default, websites / blogs can be accessed by with or without the www in the front of the domain name; however search engines see this as two different domains. One with the www and one without. To avoid content duplicity it is advisable to choose one and to stick with it, this way SE’s can consolidate all links to your domain whether they are with or without the www and give you the maximum link juice.

There are many ways to accomplish this, you can find in detail on this website. You can also use Google Webmaster tools to tell Google your preference so that you are nicely covered in the biggest search engine.