You must consider using a 301 re-write rule in your .htaccess file so that both addresses (http://example.com and http://www.example.com) resolve to the same URL. 

- If you want to redirect http://www.example.com to http://example.com, you can use this:

RewriteCond %{HTTP_HOST} ^www\.example\.com$

RewriteRule ^/?$ "http\:\/\/example\.com\/" [R=301,L]

- If you want to redirect http://example.com to http://www.example.com, you can use this:

RewriteCond %{HTTP_HOST} !^www.example.com$ [NC]

RewriteRule ^(.*)$ http://www.example.com/$1 [L,R=301]

Note that you must put the above lines somewhere after RewriteEngine On line.