It depends what your primary domain is set to, usually you just use a CNAME to redirect to the non-www or www version. I don't think you will get duplicate content issues if you have a redirect in place, Google should be smart enough to know that it's the same URL, but this won't work for subdomains. You can also inform Google of your primary domain through Webmaster Tools, or you can setup canonical URL's if you start seeing duplicate content warnings.
I would avoid using htaccess if it's avoidable, in this case it is, because you can manage the domain redirect through your DNS provider. Htaccess can cause sites to slow down because every request causes a look-up of the htaccess file, if I remember the process is something like this:
- Look up the root htaccess file
- Look for any other htaccess files in other directories
- Batch the files together
- Reconfigure the server with the new settings
Only then will you get a file sent down the wire, and it happens for every single request and can't be cached. If you've ever seen warnings about a lengthy time to first byte, this is sometimes the cause.