I asked myself once, "if the battles are to the death, what happens when neither fighter can die?". Now, both of these
guys can die, but only in certain ways and therefore we call them Immortals. They both seem to be of equal intelligence and experience
(Duncan is only a little older I think). Angel does have some cool abilities compared to Duncan but Duncan is cool in his own right.
Of course Duncan will have his sword but Angel is seen using his fists more than anyhting but lets give him a stake anyway.
Angel would smell Duncan's blood from a distance and realize that he is not normal, therefore a demon. The demons in the Buffy/Angel universe
are dumb and can be killed like any creature. So Angel will try to stab Duncan in the heart while Duncan will think that Angel is a regular human. Once
he tries to kill Angel and sees that stabbing him with his metal sword doesn't kill him, he will think that he is an immortal like the
kind he fights every week. Therefore it comes down to how they perceive the other. Angel is faster so he would kill Duncan "no problem".
He stabs Duncan in the heart and walks away thinking he is all bad ass. Duncan's immortality heals the wound quickly and he is able to sneak up on Angel to chop off his head.
This is how he kills his usual enemies and it works on a vampire just as well.
