Duke doesn't exactly ... change. He's worn this, or things very close to this, for about ten years. In-Game wardrobe changes will be tacked onto the end.
Anyway, here is the slightly odd looking official Duke-art.

Totally looks like a hermit, doesn't he? Really. He lives in the woods.
Its a tiny bit hard to tell, so I have two lovely pixiv artist's rendition that does more different colors for the separate pieces...


The coat with the gold shoulders and the long black tails is the top layer - it has no sleeves because Duke is in a JRPG and ???? fashion. (Not sure if the shoulders are actually metal or not, but if they are, wow, that's some nice weight-) The back of this one matches the front, except that the front opens.
The coat under this one is most of the collar, and has sleeves. Its about equal in length to the gold-and-black one.
His gloves have metal across the knuckles, and up at the top where they stay up. Why? Reasons. Don't get punched by Duke.
Under that is the frilly-necked shirt. It's buttons once it gets past that V-neck, and generally fits him like someone thought that his middle ought to be broader than it is. Nice dress shirt, full sleeved... (He likes his layers okay-)
Pants are red for some reason.
Boots are ridiculous. Don't get kneed by those, it might be unpleasant.
And then there is what happens when he absorbs too much magic.

Yeah.
On the left, is Second Form. Duke absorbs a lot of magic, starts glowing, suddenly misplaces most of his clothing, has an anatomy change??? as well as a color change??? and comes with the top row of shields.
We don't talk about Second Form.
On the right, after getting beaten up in Second Form, Duke can absorb power from magic weapons and turn into the Radiant Winged One (and now you all go "ooohhhh" at the username-) After some magical color inversion of the area, Duke has some much sharper looking shields, ... some .. remains of normal clothing, green eyes and dark skin. And he looks like he's leaking light from his hair rather than simply glowing, the way Second Form glows. The more distressing facet of Radiant Winged One are those gold-and-green devices. Due to some similarity in appearance to things like this



Those seven gold-and-green devices are likely to be "Heart Blastia"-styled things - which means that at least three of them are in his skin (like Yeager up in the middle shows-)
So yeah. That's what he generally wears / has worn in canon.
Brave New World, obviously Duke doesn't JRPG here. I'll haul out something later.
Anyway, here is the slightly odd looking official Duke-art.

Totally looks like a hermit, doesn't he? Really. He lives in the woods.
Its a tiny bit hard to tell, so I have two lovely pixiv artist's rendition that does more different colors for the separate pieces...


The coat with the gold shoulders and the long black tails is the top layer - it has no sleeves because Duke is in a JRPG and ???? fashion. (Not sure if the shoulders are actually metal or not, but if they are, wow, that's some nice weight-) The back of this one matches the front, except that the front opens.
The coat under this one is most of the collar, and has sleeves. Its about equal in length to the gold-and-black one.
His gloves have metal across the knuckles, and up at the top where they stay up. Why? Reasons. Don't get punched by Duke.
Under that is the frilly-necked shirt. It's buttons once it gets past that V-neck, and generally fits him like someone thought that his middle ought to be broader than it is. Nice dress shirt, full sleeved... (He likes his layers okay-)
Pants are red for some reason.
Boots are ridiculous. Don't get kneed by those, it might be unpleasant.
And then there is what happens when he absorbs too much magic.

Yeah.
On the left, is Second Form. Duke absorbs a lot of magic, starts glowing, suddenly misplaces most of his clothing, has an anatomy change??? as well as a color change??? and comes with the top row of shields.
We don't talk about Second Form.
On the right, after getting beaten up in Second Form, Duke can absorb power from magic weapons and turn into the Radiant Winged One (and now you all go "ooohhhh" at the username-) After some magical color inversion of the area, Duke has some much sharper looking shields, ... some .. remains of normal clothing, green eyes and dark skin. And he looks like he's leaking light from his hair rather than simply glowing, the way Second Form glows. The more distressing facet of Radiant Winged One are those gold-and-green devices. Due to some similarity in appearance to things like this



Those seven gold-and-green devices are likely to be "Heart Blastia"-styled things - which means that at least three of them are in his skin (like Yeager up in the middle shows-)
So yeah. That's what he generally wears / has worn in canon.
Brave New World, obviously Duke doesn't JRPG here. I'll haul out something later.