SharePoint 2013 search hides duplicate results by default. In order to view duplicate search results following the following steps.
Edit the search result page and edit the Search Results web part
Expand the Settings section and tick the box for “Show View Duplicates link”.
Save the setting and publish the page. When you search for a term, hover over a search result. In the panel that loads click on VIEW DUPLICATES.
For information on how SharePoint calculates duplicates see this blog post.