mrbenja0618 said:
But why have a character who is already named Darth Vader?

Darth Vader is just a name for the leader of the pirates, he wears armour and a mask (but no breath screen) so no one has ever seen his face (every time one of their leaders dies they put the suit on someone else) that way when Anakin is captured in episode II the first time audience can assume he is dead and that in Episode III he is fighting the same Darth Vader in the other episodes (when in fact he's fighting Anakin in the armour).

The identity of this last Vader is not revealed until ESB.

In this prequel trilogy Obi-Wan's connection with finding a Vader is made out to be personal but the full nature is never revealed so when he decribes him as a former pupil (and he has had others) in ANH that (and the apparent death of Anakin) would seem to be the reveal as to why Obi-Wan took the conflict between them so personally.