Make sure every page on your site has a unique TITLE. It's that simple. You have two or more pages on the same site that have the same contents inside <title></title>. Sometimes this happens if you have multiple ways of accessing the same page; some e-commerce systems like Magento are notorious for this, which is why you might have to blacklist some pages with a robots.txt or a robots META. The tools should be giving you a list of the pages with duplicate titles.
It's about the <link rel="canonical" href="http://www.example.com/abc" /> This link would help: https://support.google.com/webmasters/answer/139066?hl=en