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Google’s Advice On Using The New Canonical Tag
A month ago, Google, Yahoo and Microsoft announced they will be supporting a new canonical tag that allows you to tell search engines that page X is a duplicate page to page Z. In a way, it is a 301 redirect, without the physical redirect.
The tag is incredibly powerful, as are 301 redirects and using this tag should be done with caution and slowly. Matt Cutts posted a new video explaining how one should go about using this tag, being that it is so new. Here is the video: