Solving the Canonical URL Problem
Most database driven websites have this problem. Programmers and SEOs are always trying to find ways of solving the Canonical URL problem because search engines find two or more different URLs but having exactly the same content as a duplicate content.
Here is one example:
http://www.searchmarketer.ph/index.php?page=this-is-a-page
and
http://www.searchmarketer.ph/index.php?page=this-is-a-page&affID=palconit&sessionID=555ABC
can be have exactly the same content
One solution to this problem is either to force your site to show SEO Friendly URLs using mod_rewrite and other kinds of URL rewriting solutions available, just to hide this problem. But we cannot always control the URLs generated by our site. One reason is search engine spiders will always try to crawl and index any link they come across your page and your undesired URLs somehow gets discovered and indexed and then treated as a duplicate entry, hurting your site.
While reading the Google Webmaster Central Blog, they came up with a solution that we now only need to identify which page we want Search Engines (specifically Google) to identify which page URL needs to be recognized and indexed.
The code to add is:
<link rel=”canonical” href=”http://www.searchmarketer.ph/index.php?page=this-is-a-page” />
inside the <head> tag of source code.
This way search engines will recognize the one you identified in the code above as the canonical URL. More information about solving the URL duplicate content problem here.
Thanks for reading 
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