Google has introduced a link tag rel value of canonical which is used for defining the value of the page that Google should use. Why? Lets say you have an eCommerce site and one of the product URLs is:
Imagine now that you can make a slight customization to the product that changes the URL to:
Uh oh — Google sees duplicate content! The code shows essentially the same page so Google thinks you’re pulling gangsta stuff. Now you can tell Google what URL to use for the current page to avoid duplicate content penalization.
<link rel="canonical" href="http://yoursite.com/product.php?p=david+walsh+blog+book" />
Don’t let your website get penalized by Google; use this link/rel tag combination for your highly variable pages.