Madara had fought Duke before, but at the time, he had been on his last legs, with half his body already in the grave. He had not even brought his Rinnegan with him, so he had lost with great reluctance.
After being revived, Madara returned to his peak and rose to an even greater level. He finally got his wish and defeated Hashirama, who had always loomed over him. He no longer regarded anyone else as worthy of notice.
To Madara, Duke barely counted as half an opponent, a little stronger than the Five Kage. Since they did not belong to the same era, there were no bonds or bitter entanglements between them. If they truly had a grudge, it was probably because Duke had beaten up an old man back then and made him lose face.
The Shinobi World's Asura was not a forgiving man. He had remembered this grudge all along. He decided to deal with Duke personally and reclaim his honor from back then while he was at it.
First, the war was far too dull and needed some seasoning. Second, the pride of a strong man was at work. Madara craved a battle that would make his blood boil, so he skipped over the Five Kage and set his sights directly on Duke.
Madara believed Duke would not disappoint him. Thirty was the peak age for a ninja, and Duke must be monstrously strong by now. But that was fine. He was monstrously strong too, and he firmly believed he was the strongest. No matter how gifted Duke was or how hard he trained, he could never bridge the gap between them.
*Duke: You're right. On the path of training, I've always been extremely strict with myself. Even when I'm resting, I've never let up for