Ahh, it may be that the Duchy has been integrated into your kingdom so that it doesn't have any separate laws but the succession has been switched to your Norwegian one.
@Calbrenar: I'm a little confused. Are you absolutely sure they're your sons?
Absolutely. I've seen it with sons / daughters / bastards / heirs. Sometimes it works sometimes it doesn't. I've only ever seen it fail with priests I think (because I like to try to get their relationship liking over the popes so I can get taxe). Kind of ironic that catholic priests don't like you better for sending them your son