I’m still not even remotely convinced that the Duke has anything to do with anything. I think he is just a minor minor minor character who was mentioned only because they needed someone controlling Zoso and running the war.
The Duke could have some significance that we find out later on in the story, kind of like how the bad fairy that gave Snow the fairy dust may or may not be Tinkerbell. If the show ever goes into the Dark One’s history before Rumple, then we may find out how the Duke came into possession of the Dagger. We’ve got a time gap between Rumple became the Dark one to the Return where Rumple has stopped the Ogre war. I think Rumple may have killed the Duke of the Frontlands since he was the one who was sending children like Bae into battle.
All magic comes with a price!
Keeper of Felix