In OTL, during a rebellion between 1077-79, at a siege at the town of Gerberoy, William I of England is fighting his rebellious son, Robert, during a sortie from the castle at Gerberoy. In battle, the two seemed to have personally fought, where Robert unhorses his old man and nearly kills him, only to hesitate when he recognises his father's voice.
Father and son are reconciled through the efforts of Matilda, the wife and mother of the two men. But subsequent fallings out results in Robert disappearing from Normandy for years, but is acknowledged by his father on his death-bed as heir to the dukedom of Normandy, while the younger son William Rufus inherits the English Crown. Henry, the youngest of William's sons, receives a fortune from his father.
What if Robert Curthose had perished at Gerberoy? Richard, William's second son, initially would have been the considered heir until his own demise when hunting in the New Forest in 1081 (just like his brother William II Rufus nineteen years later). So then would Normandy-England be inherited by the same son? Or would the duchy and kingdom be jointly inherited by William and Henry? And who would get which realm?