William hasn't been promised any Dukedom. Either he is created a Duke in his own right the morning of his wedding, inherits a Dukedom (Edinburgh) if his father and grandfather predecease The Queen, or becomes one automatically when his grandmother dies by right of being the eldest son of The Sovereign and heir (Cornwall).
So, the reality is William is going to become a Duke one way or another even if his grandmother doesn't create him a Peer with marriage. If his father doesn't survive to be King, William will become The Duke of Edinburgh upon Philip's death. If his father succeeds to the throne, he will become The Duke of Cornwall automatically.
So, the reality is William is going to become a Duke one way or another even if his grandmother doesn't create him a Peer with marriage. If his father doesn't survive to be King, William will become The Duke of Edinburgh upon Philip's death. If his father succeeds to the throne, he will become The Duke of Cornwall automatically.