There are a lot of websites and companies that buy up multiple domains in order to try and corner the market and weed out the competition. While at Classmates.com I found a long list of domains that the company owned, some of which were parked, and others were resolving to the same DNS as the www.classmates.com domain. The problem with the multiple domains pointing to the same DNS for SEO is that the search engines view this as an exact match down to the same directory and filename, as well as the content on the page. The result in the end is that one or more of the domains could be banned or even penalized for being the same.
I ran into the same issue with the professional womens organization - the original domain was www.pwoo.org which was later changed to wwww.professionalwomenonline.com. The www.pwoo.org domain was used when I was quoted in articles and marketing publications, so I wanted those who came from there to still arrive to the same site even though the domain changed. With the two domains pointing ot the same site, I struggled with ranking issues due to links, and duplicate content. So I removed the www.pwoo.org domian from the Google index so that it won't obtain rankings anymore. The www. professionalwomenonline.com domain is now getting great rankings and the users who type in www.pwoo.org can still see the same site.
How did I resolve the issue?
Google has a great addition to their webmaster tools in which you can remove files, directories, and even whole domains. By removing one of the domains from the index, the other can be left to obtain rankings without resulting in being blacklisted. The only problem is that you have to have the pages 404 (page does not exist) in order for the domain to be removed.
Back up your files on a computer or separate server and remove them all (just temporarily)
Go to your Google webmaster tools and select the site you wish to choose (if you don't have a site setup in the Google webmaster tools I would advise creating an xml sitemap and setting up an account today)
From your webmaster tools site dashboard select the "URL Removals" for your website
Click the "New Removal request"
In this case you want to select " Remove your site from appearing in Google search results."
Confirm by clicking the " Yes, I want to remove this entire site."
The site is then added fore removal.