if your site open with two type of url like http://cogniter.com and http://www.cogniter.com its called canonical problem in SEO Google takes it as a duplicate site and some times punish it.
If you site opens with two or three types of URLs such as abc.com or abc.com/index.html this is the canonical error in your website 301 Redirect is the solution of this error !!!
Duplicate domain is a canonical error like you have a website mysite.com and it is opening with both of types means with www and without www. Thats a canonical issue. Apply 301 redirection on website.