what can create duplicate content:

canonical issues (www and non-www version);
pagination when different pages have identical titles and meta description;
various versions of the home page (e.g. www.site.com and www.site.com/index.php);
incorrect internal navigation creating several URLs to one and the same page

There are only few free tools available that can be of much help identifying your site duplicate content:

1. Duplicate content tool estimates the following:

www and non-www header response;
Google cache check;
Similarity check;
Default page check;
404 header response;
PageRank dispersion check