A concern that crops up often in the forums and helpdesk is how to handle domain pointers, redirects and anything else that could lead to your site’s content being available at more than one URL.
A lot of people have read or been told that search engines in general, and Google in particular, will lower your site’s placement in search results if your site is available at more than one URL – the dreaded duplicate content penalty!
According to Google’s Greg Grothaus, there is no such thing. Unless you’re, you know, spamming or scraping content from other sites, in which case you probably should be penalized.
But even though there is not a penalty imposed by the search engines, if you use multiple URLs to point to the same content, you may be penalizing yourself.
Lot’s of great tips in this video from Google Webmaster Help. It could be worth your while to take a few minutes to watch.