What may come as a surprise to some webmasters is Google’s recent announcement that cross domain content duplication will be allowed. Google is recognizing that there are legitimate reasons to duplicate content across different websites.
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For example if you are migrating to a new domain name and using a Web server that cannot perform server-side redirects. To help those webmasters with these types of issues that arise on sites, Google has created the cross-domain rel=”canonical” link element.

Google recommends following certain guidelines. Such as choosing your preferred domain, when Google is confronted with duplicate content they will generally index just one version and filter the rest out. This allows you to make the decision of which version should be indexed. In addition they recommend that you reduce in-site duplication, by placing the appropriate no index statements.

You should also enable crawling and use 301 (permanent) redirects wherever possible. These type of redirects send visitors and search engine crawlers to your preferred domain and make it extremely clear which URL the search engine should index.

Finally the use of the cross-domain rel=”canonical” link element; this is to be used in situations where it is difficult to set up a redirect.  Like the previously mentioned situation where a server does not feature server-side redirects. You would use the rel=”canonical” to specify which URL of the domain is preferential for indexing by the search engine.

For more on this visit http://www.kingpin-seo.co.uk/webmaster-magazine/google-announces-cross-domain-duplicate-content-tag-kingpin-seo-agency-news/01273