David Whitehouse has a great SEO guide talking about how to deal with duplicate content on your site. I consider this a must read for anyone that deals with websites and online publishing. Anyone who knows me knows that I'm anal about duplicate content and will preach about 301s and canonical URLs until the end of time.
In a nutshell: no particular public resource, article or page on your website should be accessible via more than one unique URL. Make sure your site 301's to either www or no-www, pick one and stick with it. I love no-www. Unless you have to use an Amazon load balancer that requires www, then no-www should also be your pick.
David's post goes into more detail so there's no point for me to rehash it here. Do yourself a huge favor and make sure you abide by these best practices.
And while I'm on the subject of SEO, I thought I should mention that my friend Evan Bailyn just published a book: Outsmarting Google: SEO Secrets to Winning New Business. I received my copy today but have yet to read through it. Between that, Gary Vaynerchuk's The Thank You Economy, Bargaining for Advantage and some design books, I have lots of reading (and not enough time) cut out for me.