A common rookie mistake for setting up a web site is to have two or more domain names that you own all resolve properly for the same web site.

Now Google is notifying webmasters via Google Webmaster Tools if they spot this issue. They are sending notifications that your site is “served by two domains” or more.

For example, people often confuse this site with SEOroundtable.com or searchengineroundtable.com when it really is seroundtable.com. Rookies would have all three domain names resolve and work fine, which would create two canonical versions of this site and major issues in Google.

The email from Google reads like this:

I believe this message has been around since April 2012 but maybe earlier.

Forum discussion at Google Webmaster Help.