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For some sites, there are legitimate reasons to duplicate content across different websites — for instance, to migrate to a new domain name using a web server that cannot create server-side redirects. To help with issues that arise on such sites, we're announcing our support of the cross-domain rel="canonical" link element.
But if a 301 redirect is impossible for some reason, then a rel="canonical" may work for you.
While the rel="canonical" link element is seen as a hint and not an absolute directive, we do try to follow it where possible.
There's a companion post, from October 06, 2009, about reunifying duplicate content within your website [googlewebmastercentral.blogspot.com], which Mueller recommends reading first.
In both posts, he emphasizes that, in lieu of a 301, the rel="canonical" link is better than blocking the page....
One item which is missing from this list is disallowing crawling of duplicate content with your robots.txt file. We now recommend not blocking access to duplicate content on your website, whether with a robots.txt file or other methods.
Read fine print carefully with regard to which blocking methods you shouldn't use, depending on which kind of duplication you're fixing, as blocking the page will conflict with the rel="canonical" link element.
One thing we do know, the canonical redirect must point to a URL with "substantially similar" content in order to kick in. That eliminates a bunch of potential trouble.
more reason not to allow HTML in user-generated content
the canonical redirect must point to a URL with "substantially similar" content
Keep in mind that we treat rel="canonical" as a hint...