I'm still working out my feelings regarding the Duncan in GEoD, but I'd like to point to a couple of passages.
She moved a step closer. He smelled the milky warmth of her breath.
“They made me too well,” she said. “I was designed to please an Atreides. Leto says his Duncan is more an Atreides than many born to the name.”
Idaho swallowed. When he thought of Hwi, the emptiness in his breast became full. The message of that could not be ignored. Something of terrible power bound him to Hwi.
He tried not to think about Hwi. Rage threatened to engulf him when he did. He remembered the Fremen word for that intense emotion: kanawa, the iron ring of jealousy. Where is Hwi? What is she doing at this moment?
This Idaho has clearly fallen victim to Hwi's charms -- and we know that the Ixians created her for just this purpose. Even Moneo becomes enchanted with her after being exposed to her more.
How calm she is, Moneo thought... Hwi was exactly what she appeared to be--an ultimately sweet and sensible human being. Would she really have mated with me? Moneo wondered.
A page later, Hwi and Leto are discussing Moneo's recent behaviour.
"He has been very moody and distracted since you returned from the Little Citadel. He's not the same at all."
"He is an Atreides, my love, and you were designed to please an Atreides."
"It's not that. I would know if it were that."
Anyway, my first point is that Duncan seems bound to Hwi similar to the way Leto is (though, Leto's bond and need is much stronger). I don't think they can really control these feelings of love for her, and I think they both know this. They are, in a sense, powerless and vulnerable with her. Luckily, she is as pure as she seems, and therefore does not wish to exploit this fact.
My second point is that Duncan shows signs of real maturation, and is not, in fact, as childish and petulant as he seems. I draw attention to the scene in Moneo's office, where he summons Hwi and Duncan for a discussion. Duncan comes off looking pretty nasty in this scene. He doesn't even try to conceal his contempt, and even displays anger towards Hwi.
First, note that at the beginning of the dialogue, Moneo completes one of Idaho's sentences, leading to the following exchange:
Idaho shook his head. "Does everyone here know what I'm going to say before I say it, what I'm going to feel before I feel it?
"One of your admirable qualities," Moneo said. "You do not conceal your feelings. We--" he shrugged--"are necessarily more circumspect."
This is important because, as we see at the end of the chapter, Duncan is behaving exactly opposite to this.
Anyway, over the course of the conversation, Duncan gets worked up and eventually storms off.
"Damn you all!" Idaho snarled. He turned and dashed from the room.
And finally, the proof.
He strode the few paces to his door and into his quarters.
Once inside, Idaho stood a moment with clenched fists at his sides.
I have no more ties to any time, he though. And how odd that this was not a liberating thought. He knew, though, that he had done the thing which would begin freeing Hwi from her love for him. He was diminished. She would think of him soon as a small, petulant fool, a subject only of his own emotions. He could feel himself fading from her immediate concerns.
And the poor Moneo!
Idaho sensed the shape of the things which had formed the pliant majordomo. Duty and responsibility. What a safe haven those were in a time of difficult decisions.
I was like that once, Idaho thought. But that was in another life, another time.