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
2. Xenu scans all your site links and returns a table of all available URLs – all you have to do is to sort the list by title and find pages with identical titles.
3. Google Webmaster Tools reporting your site duplicate titles and meta descriptions.