Google, Yahoo, Ask and Microsoft announced support for a new link element to clean up duplicate urls on sites. We add HEAD part of document, its syntax is pretty simple
&lt;link rel=&quot;canonical&quot; href=&quot;http://www.webmastersucks.com/pages.html&quot; /&gt;
This syntax is telling search engines that the preferred location of this url is http://www.webmastersucks.com/pages.html instead of http://www.webmastersucks.com/pages.html?PHPSESSID=293JA434MFNs
yoast.com has already produced several plugins for WordPress, Magento and Drupal. You can find plugins in Yoast.
If we used “canonical”, then our content is not dublicated. These are same, and search engines see the same content. Make all the non-canonical urls do a permanent (301) HTTP redirect to the canonical/preferred url.
Links About Canonical Link Element