Duryodhana and Karna
Duryodhana never got to know the actual lineage of Karna. Except Kunti and Bhishma no one knew the reality of his birth till the war ended. After the war when Karna's lineage was revealed by Kunti to Pandavas, they were shocked and sad but Yuddhisthir was the most in pain. Being Dharmaraj he felt sinful for fighting and killing his eldest brother because in that era an elder brother was equivalent of father and in that era, parents were greatly respected and loved in Indian culture. Then, those who were alive eventually got to know the reality as Yudhishthir invited Karna’s wives and his remaining family for his last rites.