There is a popular myth about duplicate content, and that is that Google penalizes sites for having duplicate content.
Greg Grothaus of the Search Quality Team of Google cleard that Google itself is not penalizing you for it.
Google recognizes that most duplicate content is not created to be deceptive. There are of course exceptions, which are considered spam. They're being penalized for being spam but not for having duplicate content.
There are some issues that can arise that may negatively affect your rankings.
1) Your link popularity will be diluted. Backlinks pointing to several different URL versions(like domain.com, www.domain.com, domain.com/index.htm) of the same content, will make it harder to accumulate link juice for one URL.
2) User-unfriendly URLs in search results may offset branding efforts and decrease usability as well.
3) With multiple versions of the same thing, Google will spend more time crawling the same content, meaning it will have less time to go deeper into your site, and you run the risk of having content not get indexed.
More interesting reading at webpronews.com/topnews/2009/09/16/google-busts-the-duplicate-content-myth