If you have several versions of the same page or many pages with the same content, tell the search engines about it with the new canonical tag!
The search engines really don’t like to find the same content on several web pages. The main problem is for them to determine what is the “primary” page, the most important version, the one people should link to or the first “copy”.
The problem is often caused by content publishing systems that generate several URLs or web addresses for the same page.
Now the big three have agreed on a tag you can add to specify your preferred version of a URL found on your web site.
Here’s the syntax as explained by Google. Add this tag to all versions of the same content inside the <HEAD> part of the pages:
<link rel=”canonical” href=”http://www.examplesite.com/product.php?item=page-identification” />
Google says that although they are not obligated to follow your tag, it is a “hint” they will take very seriously.
This tag only works for pages on the same domain:
“To migrate to a completely different domain, permanent (301) redirects are more appropriate. Google currently will take canonicalization suggestions into account across subdomains (or within a domain), but not across domains.”
Yahoo! adds that the link tag will be treated similarly to a 301 redirect, in terms of transferring link references and other effects to the canonical form of the page.
The tag won’t help you get higher rankings for — let’s say — an article that is published on various sites, out of which some are not under your control. In that case, the search engines will — as before — try to determine preference based on publication dates and the “authority” of the sites in question.
Specify your canonical (Google Blog)
Fighting Duplication: Adding more arrows to your quiver (Yahoo! Search Blog)
Partnering to help solve duplicate content issues (Live Blog — site down at the moment of writing)