Two URL of a same website

I search my website visiblenews.com on google & i get through a website named statscrop.com. which syas that your seo is bad because visiblenews.com and www. visiblenews.com are showing the same page. If duplicate content in seach index can hinder your pages' performance in search results. As a result, the ranking of the site may suffer, or the site might be removed entirely from the Seach index, in which case it will no longer appear in search results. please help me..

See original post by Rajan Doomra