A new tool has been added to your arsenal to help fight against duplicate content on your site, and Google, Yahoo and Microsoft are all on board with it. Introducing the “Canonical Tag”.
This new tag allows you to specify the desired URL for a page that might have multiple URLS. For example, you might have:
All of those are the same page, but which one should the search engines index? You can use the <link> tag in the header of the page to make it clear.
In this case, you’d probably add:
<link rel=”canonical” href=”http://www.example.com/products/widget/” />
The link will be treated much like a 301 redirect, though you can’t link to pages on another domain.