In the world of dynamic sites generally, and, of course among the webmasters runing Joomla sites there is often a lot of worry about duplicate content penalties since so many different URLs can point to the same exact page.
These worries have been somehow minimized recently.
At the recent SES (Search Engine Strategies) in Chicago (December 2006, more exactly), Vanessa Fox of Google stated in an interview with WebProNews that there really are no duplicate content penalties for pages within your site. She even went as far as to specifically address dynamic sites and their content.
It is an extremely informative interview and great insight into some Google processes. She also goes over Google Sitemaps and how they can help your site get spidered faster. Highly recommended from any Joomla webmaster!
To view the full interview visit webpronews.com.