Google does not specifically penalise for duplicate content, but it does not really like it either! There are several reasons why a site might have duplicate content, including selling a particular product which is listed in the same way across a number of sites.
Duplicate content is basically any written word on the site which appears on more than one URL. It may be that a web author has copied directly from another site (either deliberate or accidental) or it could be that the site is creating printer friendly versions of pages.
The reason it is so important to avoid duplication is because search engines cannot index pages which are identical across different URLs – they will simply pick one over another to display. This can have devestating consequences for your ranking. Instead of duplication, try writing entirely unique pieces in a niche which only you can explain about in detail. Ensure that titles and headings are not too similar and use canonical tags where needed to indicate which pages should be linked to a certain keyword.