Duplicated content is a substantial block of content that appears on more than one web page (within the same domain or across different ones). The problem with duplicate content is that when a search engine bot finds it, it will not be able to determine which domain has more relevance on the subject.
But duplicate content is also a common occurrence within a page. For example, if you have a separate mobile or a local version of your website. In this case, you should pick a page that will be the original and add a piece of code (rel=canonical) to all the others, clearly pointing the search engines to the original version.