In a historical context, it's common to refer to members of the aristocracy solely by their title: hence, the Duke of Somerset will be referred to as 'Somerset', the Earl of Oxford as 'Oxford' and so on. It's only if there's some ambiguity that other names will usually be used.

This sort of thing can get confusing at times, however. During the period 1543-1553, John Dudley was successively Viscount Lisle, Earl of Warwick and finally Duke of Northumberland. Reading about the exploits of 'Lisle' in 1544, then of 'Warwick' in 1549 and the execution of 'Northumberland' in 1553, students don't always realize that they're one and the same person.