Google and duplicate content issues (www and no-www)

[grandma genie]

Hi,
I understand that all websites basically have two names. One with the www and one without. I also understand that Google considers that as two sites, so charges the site with a duplicate content penalty. Is that true? Why? We really have no control over that system. My site is a domainname.com on the site registrar's records, but it can appear as both www.domainname.com and domainname.com in the SERPS. You can't really register a www name, it just comes with the territory. So, why does Google treat it as two sites? I'm sorry for my ignorance, but this just doesn't make sense. I can understand two totally different named sites with the same content as being a problem, but not one site using the www or not using the www. Can someone please explain this? By the way, I have handled this in my htaccess file, so the problem is moot, but I just would like to understand why it is even an issue.

[enigma1]

but it can appear as both www.domainname.com and domainname.com in the SERPS

No, you control that, because if say you want example.com instead of www.example.com you force a redirect to anything that has www. in front into example.com. As you mentioned you're handling it with htaccess already.

When you register a domain you register the root and then you may have sub-domains. In essence www.example.com is a subdomain. you could also have forums.example.com and blog.example.com etc. Also your domain may be accessed by the IP itself at port 80 and with an SSL at a different port, like 443. So again you may have to configure the server so everything without a domain it is handled.

For my domains I also check the HTTP_HOST for this. So if it doesn't match the configuration of the application I issue a 301 redirect. Because again it depends how the sub-domains are configured. For instance you could physically have the forum in:

example.com/forum/
but you want it to be accessed as
forum.example.com

I find it easier to resolve these issues at the application level so the code is not dependent on the server configuration. Because with different servers you may have to write a different configuration script. htaccess is used with apache.