Although Google is great at determining which site to credit the duplicate content to they can sometimes be mistakes which means your main site may be filtered out of the search results. For some websites there are however many legitimate duplicate sites, you may have caused a duplicate site for the following legitimate reasons

Having another site for another market
Migrating sites and not being able to redirect your old site to your new one

Google has released the rel=canonical tag to try and combat this duplicate content issue, this tag however …

If you own a blog you are probably very familar with users on the Internet trying to add links and spam into your comments field. This can be very frustrating, and can often leave blogs with unrelated content and links if they are not moderated properly.
Google however have recented stated through their Webmaster blog that abusing comment fields in websites by adding un-related comments containing links can actually harm both websites rankings. Webmasters who abuse blogs comments fields with unrelated content will now actually be penalise by Google, great!
This statement …