when did this title originate?
The first recorded use of the title duke ("dux") is in an act in favour of the Abbey of Fécamp in 1006 by Richard II.
Duke of Normandy
The first recorded use of the title duke ("dux") is in an act in favour of the Abbey of Fécamp in 1006 by Richard II. 
what were the results of the norman conquest William's force defeated Harold, who was killed in the engagement who led the conquest? Duke of Normandy later styled William the Conqueror. where was the duke from? Duchy of Normandy in north-western France.
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Duke of Normandy [SEP] History of the title
The French chancery did not regularly employ it until after 1204, when the duchy had been seized by the crown and Normandy lost its autonomy and its native rulers.