Hi Gerben,

the data source for K2 items is managed automatically, never use "Manual menu item alias for SEF link".
The routing for fully SEF links is done automatically by the K2 router, thus no need to change this kind of settings.

About duplicates, i'm not sure what you mean. Normally on your site you should have a single link for your K2 items and thus in the sitemap. The K2 router should always route links to a single entry point if you have correctly configured menu items to link K2.

By the way, if you want to limit links in the sitemap to a specific category you need only to add a filter by category ID, in your case 13, see the below image.

Cheers,

John