Doppelganger is a Medium monstrosity with unaligned alignment and Challenge Rating 3 in the 2014 SRD. This stat block includes AC 14, 52 hit points, speeds, traits, and actions.
Multiattack - The doppelganger makes two melee attacks.
Slam - Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) bludgeoning damage.
Read Thoughts - The doppelganger magically reads the surface thoughts of one creature within 60 ft. of it. The effect can penetrate barriers, but 3 ft. of wood or dirt, 2 ft. of stone, 2 inches of metal, or a thin sheet of lead blocks it. While the target is in range, the doppelganger can continue reading its thoughts, as long as the doppelganger's concentration isn't broken (as if concentrating on a spell). While reading the target's mind, the doppelganger has advantage on Wisdom (Insight) and Charisma (Deception, Intimidation, and Persuasion) checks against the target.