To avoid situations in which two pages with identical content will be indexed on different URLs, one of the pages should be added a canonical tag and be defined as a canonical URL. The canonical URL is the authorized version of the page and it will be indexed by the search engines.
Examples of URL addresses with identical content:
http://www.website.com
http://www.website.com/index.html
http://www.website.com/index?tracking-code
