Answer:
More often then not, a doppelganger will provide sympathetic company, and they are prepared to listen and give advice to humans, either implanting ideas in their heads, or a sort of osmosis. It is said to be bad luck if it is seen, and rarely a doppelganger will make itself visible to friends or family, often causing great confusion. Doppelgangers can be mischievous and malicious if they wish.