Handling Legitimate Cross-domain Content Duplication @ Google Webmaster Central

We've recently discussed several ways of handling duplicate content on a single website; today we'll look at ways of handling similar duplication across different websites, across different domains. 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.

Ways of handling cross-domain content duplication:

Choose your preferred domain
When confronted with duplicate content, search engines will generally take one version and filter the others out. This can also happen when multiple domain names are involved, so while search engines are generally pretty good at choosing something reasonable, many webmasters prefer to make that decision themselves.

Reduce in-site duplication
Before starting on cross-site duplicate content questions, make sure to handle duplication within your site first.

Enable crawling and use 301 (permanent) redirects where possible
Where possible, the most important step is often to use appropriate 301 redirects. These redirects send visitors and search engine crawlers to your preferred domain and make it very clear which URL should be indexed. This is generally the preferred method as it gives clear guidance to everyone who accesses the content. Keep in mind that in order for search engine crawlers to discover these redirects, none of the URLs in the redirect chain can be disallowed via a robots.txt file. Don't forget to handle your www / non-www preference with appropriate redirects and in Webmaster Tools.