The trouble with owning multiple domain names and pointing them at the same content
In the early days of the web it was a good policy to hold multiple domain names for one web site for copyright, or domain branding purposes, that’s no longer the case – multiple domain names pointing to the same content is now a negative because it causes perceived duplicate content.
Google determines content and it’s related ranking based upon the unique domain name that information resides within. If two domain names have the same content Google tries to determine which of the two domains is either more authoritative, or more established. It will then give a higher rank to whichever domain meets their criteria.
The reason for this process is Google’s desire to provide only the most relevant and most authoritative information possible to someone searching for that information.
So when two domain names point at the exact same content in the same location, chaos can ensue. It’s not unusual for some or most of the content to show up in Google under one domain, but have other portions of that content show up under the other domain.
When this occurs, the content and pages that show up under the second domain’s name, that content is discounted at the first, and thus the page ranking and authoritative weight of that first domain is negatively impacted – less content is associated with that domain, less pages.
This holds true whether you have to unique and different domains, such as www.site1.com and www.site2.com or you have site1.mysite.com and site2.mysite.com – Google sees these all as unique domains.
And to a lesser degree, the same problem can occur when you syndicate content – even just one page of information from your site out onto the web at other web sites. Vanessa Fox has a great blog post at her web site that specifically discusses duplicate content issues with syndication, and how you can avoid that problem.
If you have the need to own more than one domain name for copyright or branding ownership reasons, then only one domain name should be the primary. All other domain names should be set up so that they are configured with a 301 permanent redirect applied, pointing toward the primary domain. This allows you to have several domain names, but tells the search engine that only one is the currently accurate location of your content, and thus the authority domain.