The best choice in this quest is to Kill the Doppler. Even if you will receive more crowns when you spare the doppler, this is the only way to get the doppler mutagen and the doppler trophy.

Are dopplers good in The Witcher?

Dopplers are exceptionally kind and gentle beings in general, for whom the ability of metamorphosis serves as compensation for lack of aggression, fangs and claws.

Should you kill the succubus Witcher 3?

While she won't be pleased if you tell her she has to leave, she'll nonetheless give you the Succubus trophy to show as proof and Maugrim. The door will also automatically unlock for you. If you instead choose to kill her, you can loot the body for the trophy and a Succubus mutagen, but no sword.