The most common trick among WordPress bloggers is to block search engine bots from indexing duplicate content by disabling the entry routes through robots.txt. The robots.txt method works but there’s another simple method – switch from the_content() WordPress template tag in multiple post view template files to the_excerpt(). The post tag will reduce the content to excerpts, thus minimizing duplicate content. Thanks Lorelle.
Prevent Duplicate Content Posts in WordPress Blogs
How to Prevent Duplicate Content in Wordpress sites without chaning the robots.txt file.