Sometimes, using a content management system like Joomla, Drupal, or WordPress can present an eye-raising duplicate url issue.
This can happen when url strings and sef urls are indexed. Another such possibility is when a category or section is appended to the page; Thus, this is 2 separate urls. In many cases, Google will pick one.
Here is a good page regarding duplicate urls with Joomla: Joomla and SEO