buckworks - 4:21 am on May 28, 2013 (gmt 0)
It sounds as though that other domain is pointing to your server's IP address.
That can happen by accident when someone takes a site down but leaves the DNS as is, and it can cause your content to show up on their domain name if your site happens to be the default for that IP address.
It's a problem when it happens, but it's not usually malicious. There's a good chance the other domain owner doesn't realize it's happening, and there's also a good chance they'll update their DNS if you contact them with a polite request.
Check the name servers for the domain that is mirroring your site and see if you find any clues there.
You could also ask your host to make sure your site is not the default for the IP you're on.
This situation happened to one of my sites a while back but once friends helped me figure out what was happening (hat tip to Martinibuster), I was able to get the problem solved within a day or two.