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Duke was the abandoned one. I don't remember who killed their parents but it was probably his brother. His brother also killed himself for duke to be safe
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Indeed, the duke was the abandoned one. Once the late duke realized that the kid he abandoned was perfect to be the next heir and the kid he kept was a 'softie' or not fit to be the next duke, he brought him back. The twin then killed the late duke and duchess to protect the current duke from them, but he also killed himself so he wouldn't be an obstacle for him.
I think the Duke was the abandoned one and then he killed his parents and his brother to protect him killed himself so others would think he was the Duke