This site is probably scraping your site and some code on your site is redirecting back to you which is unintentional on their part. What you need to do is find the IP address of the other site and block it using htaccess. That will prevent them from being able to access your content. Once you do their site should literally disappear immediately and their search rankings should fall away soon.
Here's what the htaccess would look like assuming an IP Address of 188.8.131.52:
deny from 184.108.40.206
allow from all
I would also add canonical URLs to your pages just to make sure Google knows you are the original author of that content.