Vanessa Fox from Google Engineering answers a couple of questions in her post. Here is an excerpt-

Why do I have to add my Sitemap file to my Google Sitemaps account? Can't I just link to it from my site?

If you aren't yet indexed, submitting a Sitemap file lets us know about your site.

My site is indexed using both the www and non-www versions of the domain. How can I fix this?

This can happen when both versions of the domain (for instance, http://www.example.com/ and http://example.com/) point to the same physical location, and links to your site use both versions of the URL. If your pages are listed under both versions of the domain, don't use our URL removal tool to remove one version of the pages. Since the pages are at the same physical location for both versions of the domain, using the URL removal tool will remove both versions from the index.

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