I have only tried Mid, but I found doppels scattered randomly in forest areas in Odin'd Gate near the docks and among the ice giants and ghostly invaders, and also sprinkled through the maze in Jamtland .... and one standing by a stone in the middle of the road to Jamtland. They mostly con red and sometimes spawn a Cthonic Crawler spider (yellow to 50) but seem to go down easily enough ... not especially tough or easy.
The quest giver has a list of rewards---mostly potions.
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The quest giver has a list of rewards---mostly potions.