Hi,
I heared that if i don't redirect the user from http://domain.com to http://www.domain.com it will causes duplicate content of my website on google & other search engines, is that happening? and What is the Mod rewrite Syntax to do it through .htaccess ?
Thanks
google (and other big searches) usually detects "full clone", especially without www. prefix. You can register site in google webmasters and select, will google use www. prefix for your site, or should it use only domain.com.
According to Google themselves (see the last paragraph), you don't have to worry about content duplication at all unless you're doing it on purpose to trick search engines.
Check out Google Webmaster Tools. It's basically an interface for domain owners to control how Google indexes the site. There you can set up what's called a "Preferred Domain" which means that when Google will translate links from any of the DNS subentries to a single DNS name (i.e. example.com => www.example.com) and will not direct people to the others.
htaccess alternative, rel canonical
<link rel="canonical" href="http://www.domain.com" />