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No its not. one should redirect to the other. but I think your problem is that you have links within your site that point to both, this is why the auto sitemap generation generates both. find the internal links and fix them.

Thanks Matt, that explains it.

With the army of new TLD's coming out. We will have to see how much weight (or lack of) Google will place on them. But from previous actions, the top TLDs .com/net/org will carry more weight than usual in terms of Exact or partial match domain.