Hare Krishna Jaswant,
The Kali who was Duryodhan is not the same sinful Kali (predominating deity of Kali yuga). Kali (Duryodhan) was a great devotee of Lord Sankarshan in His previous birth. Only to show the world what happens when one becomes averse to Lord Krishna, Kali took up such a bad role (that of Duryodhan). However, this Kali is actually a pure devotee of Lord Sankarsan, which we can actually note by seeing His devotion to Lord Balaram when He was Duryodhan. This is all clearly explained in Garga Samhita.
Your servant,
indulekhadasi