A doppelganger is a creature who takes the form of another, usually the one it is attacking.
Castlevania Chronicles
The doppleganger boss is fought in two areas in the game. The first fight is in the Underground Waterway, where his weapon is a mockery of the Vampire Killer (the copied whip glows red) and has a blue body. The second fight is before the second floor of The Pagoda of the Misty Moon. This time it glows red, has more HP and attacks with it's fists.
Castlevania: Symphony of the Night
Doppleganger10 Videos: Library Tactics Luck Mode Vs Richter