When content on two or more pages is the same or substantially similar, this is known as duplicate content.  While duplicate content could be viewed as a black hat SEO technique, usually duplicate content is non-malicious (or normal).  Reasons for duplicate content could include:

• Discussion forums where content is repeated or stripped down for confersation flow
• Inventory items that are shown or linked through a number of distinct URLs
• Printer versions of web pages
• Information or news feeds 

Google understands this and attempts to deal with duplicate content is a way that does not penalise the site owner when the duplicate content is normal to the site.  Google also advises that there is no such thing as a duplicate content penalty on a site.  Although duplicate content may dilute the site ranking results (if the pages compete) and may lead to a poor user experience, and therefore should be avoided.  However,there are penaltiesfor having the same content as another site—for example, if scraping and republishing content from other sites without adding any additional value. These tactics are discouraged in the Google Webmaster Guidelines.

Different pages on the same site that contain substantially similar content can be dealt with through canonical links.