The use of "the Conquerer" ensures that the reader will know who is being referred to.

As does the use of the number, which I believe is Jazzo's point. Written as "William I the Conqueror" it bothers me, but it is important to include both 'I' and 'the Conqueror' togather at least once. I would have written it "William I, the Conqueror" or "William I, known as William the Conqueror". It is redundant only if you already know that Will I and Will the Winner are one and the same, which you probably wouldn't if you were looking him up in an encyclopaedia.

Edit: Yeah, like Max said.