Although I've got the plugin to create well formed XML, it is throwing up the following error when tested on Google webmaster tools:
This URL is a duplicate of another URL in the sitemap. Please remove it and resubmit.
I'm not sure if this is an easy one to fix, as the plugin doesn't appear to search on a url-by-url basis, and can't remove any duplicate entries.
Udinra - can you confirm whether fixable?