— Conversation overheard on the streets of Novigrad
The imp pestering the merchants of Novigrad turned out to be a doppler. These monsters usually avoid confrontation – to everyone's great relief, for when pinned they become extremely dangerous. Fighting them quickly turns into a fight against oneself. Or, to be precise, against a mirror image gifted with one's own strengths and abilities. In a conflict with such even odds, only luck – or destiny – decides the victor.
Geralt has an extra helping of both, and so won his fight against his doppler doppleganger – then decided to let him go free, for unlike other monsters, this creature's motives for harming humans were noble.