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.
I forgot to say, welcome to Webmaster World! :)
They both are on IP address: 184.108.40.206 but do not share common DNS servers.
I would contact your hosting provider for a fix.
[edited by: bill at 5:36 am (utc) on May 28, 2013]
[edit reason] obfuscate IP address [/edit]
welcome to WebmasterWorld, shumi31!
as buckworks suggested it could be DNS configuration problem.
do an nslookup for both hostnames and see if they resolve to the same IP address.
otherwise it is possible that the "offending" site is getting the content by using a reverse proxy.
Thank you so much to all of you. Already we have solved this problem. But still there is a problem that when i am typing http://example.com , it is redirecting to http://www.example.org . Why its happening?
nslookup example.com will get you the IP address of the web server.
when you get there look in the .htaccess file (in the document root directory) for some external redirect directives with www.example.org as the target.
you shouldn't be able to view this file from a web browser.
Agree with phranque, NSlookup of the domain example.com can shed some light on it. There you can find if there's any 301 redirect present at .htaccess. Or else if you have access to the hosting account or Cpanel then you can directly use the ,htaccess file to check if there's any 301 redirect present in the file
welcome to WebmasterWorld, monitorscout!
By using cross-domain canonical tags two site can have similar content. For example x.com and x.co.uk. After modification if its not fixed then ask to the webmaster where you host your site there may be some mismatch happen to x.com and x.org.