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:
We noticed that you have very similar pages served by both example1.com and example2.com (for example, "http://www.example1.com/filename.html"). Google is currently showing the example2.com version in search results. For more information about this message and what you can do if you prefer the other way, please see this Help Center article.
Message ID: XXXXX.
Please include this ID in any messages you post in ourHelp forum.
Sincerely, Google Search Quality Team
I believe this message has been around since April 2012 but maybe earlier.
Forum discussion at Google Webmaster Help.