Advice on Preventing Duplicate Content
Improve SEO score for your site
How do you add a canonical link element (aka a canonical tag) to your website? Actually, the whole process if pretty easy. As a disclaimer, each canonical tag should reference an individual page, so each page of your website should have a tag that is specific to that pate, or else bad things could happen.
http://www.mattcutts.com/blog/canonical-link-tag/
To install a canonical tag, just install the following code (with the page name replacing the sample text) in the head tag (near the metatags) of your source code for each page.
<link rel="canonical" href="http://www.sample.com/sample.htm"/>
Why do people use canonical tags? They may have several pages with substantially similar or duplicate information, but need to make sure the search engines only index a single page or the “right” page for what the webmaster wants a viewer to see. If you have a printer friendly page, a PPC landing page, or a product description page that all use the same block of text as the “main” page containing that content, the canonical link element is the way to go.
Another reason to use canonical tags is when you are flagging or tagging pages with URL parameters for tracking purposes. In some cases, search engines will see these pages as their own entities, so every variation in parameters can present a new duplicate of the main page. Canonical link elements can make sure that search engines like Google ignore the pages that aren’t intended to show up in the search engine results.
You can also use the canonical link element on other websites that show the same content. The code for the “target” page is what you put in the source code of the other pages, whether they are on or off the site containing the important page. Now, content for those pages is correctly attributed to your preferred site.
Should you put canonical link elements on the pages themselves, so the element is self-referencing? Yes. You never know if someone is going to steal your source code, accidentally point their DNS settings to your server, or do a myriad of other things that can cause SEO headaches. By having a self-referencing canonical tag, you can ensure that carbon copies of your pages are still giving credit to yours.
There is some speculation as to whether canonical link elements pass pagerank to the intended target. Several people in the SEO community support this idea. It makes sense for search engines to attribute link popularity to a “canonical” page, and not to a bunch of different versions of the page, but search engines don’t always behave in ways that make the most sense for users, so you should still check with webmaster tools to ensure that most inbound links point to the best possible page.
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Notes and Special Information
Special note: Not all search engines recognize the canonical link element. Yahoo may ignore it. Search engine optimization advice may become outdated very quickly, and search engine placement tricks become more refined over time, so make sure you do your research when applying additional SEO tactics to a site.