For anyone has been involved in international SEO the issue of cross domain duplicate content is quite a big one. Well now Google have introduced a new alert in Google Webmaster tools to alert you when they don’t show a URL because it has been duplicated elsewhere. At the moment a webmaster can resolve some of these issues using a cross domain canonical-tag or by simply 301 redirecting a page. But you may never know about half the traffic being lost due to this issue. The main causes Google give for a cross-domain URL selection (when they choose another URL over yours) are:
1. Mutli-Regional Websites : Having identical content across your regional websites. This is one of the most common reasons for this type of alert. It’s also one that can’t be easily solved by the canonical tag or 301 redirects as you want the person to land on the proper regional site.
2. Configuration Issues: Where you may have misconfigured a plugin so the canonical tag on your site is set to target external pages and Google will choose those over pages from your site.
3. Malicious website attacks: You know the kind, when your site is hacked and a canonical tag is inserted to point at external pages.
You can read more over at SearchEngineLand as well. This is a really nice addition from Google who continue to make GWT and GA incredibly powerful tools for webmaster s.