Google announced today that it is launching new Webmaster Tools messages to notify webmasters when its algorithms select an external URL instead of one from their website, in duplicate content scenarios.

Google defines “cross-domain URL selection” as when the representative URL (the URL representing the content in a duplicate content scenario that Google’s algorithm decides to use) is selected from a group with different sites.

In other words, the selection Google goes with when two or more sites are showing the same content.

“When your website is involved in a cross-domain selection, and you believe the selection is incorrect (i.e. not your intention), there are several strategies to improve the situation,” says Google Webmaster Trends Analyst Pierre Far.

Google highlights three main reasons for unexpected cross-domain URL selections: duplicate content including multi-regional sites, configuration mistakes and malicious site attacks. Far points to various resources for each scenario in this blog post.

“In rare situations, our algorithms may select a URL from an external site that is hosting your content without your permission,” says Far. “If you believe that another site is duplicating your content in violation of copyright law, you may contact the site’s host to request removal. In addition, you can request that Google remove the infringing page from our search results by filing a request under the Digital Millennium Copyright Act.”

At least the WMT messages should help alert you when it’s happening.