magaworks, I ran into this same issue. You need to add a ‘/’ in front of your custom wordpress feed file. So this:
load_template( TEMPLATEPATH . 'your-custom-feed.php');
becomes:
load_template( TEMPLATEPATH . '/your-custom-feed.php');
I was having this same issue. I tracked it down to a plugin called: Slash Comments which was adding the duplicate attribute in the xml. Deactivating the plugin fixed the issue. It looks like the functionality that the plugin served has been added to the WordPress Core code.
Thanks for the info. It worked perfectly for me!
