Duplicate content is a misleading term. When you first happen upon it, it seems to imply that your site has the same content on more than one page. Or that your site is duplicating (or has stolen) content that’s already on someone else’s site. But in most cases, when someone says your site has duplicate content, they mean that more than one url pointing to your website serves up the same content. For example, it’s not uncommon for the following url patterns to return the same page:
www.mysite.com/mystuff/
mysite.mystuff
www.mysite.com/mystuff/index.php
mysite.stuff/index.php?some-parameter=some-value
The confusion is compounded when you have more than one domain name pointing to the same site.