Unidentified doppelganger

This doppelganger infiltrated the human Catyurit family and began poisoning the produce of Emon.

Abilities
Presuming the doppelganger has the typical traits of its race, it has the following abilities:
 * Condition immunities: The doppelganger cannot be charmed.
 * Senses: The doppelganger has darkvision for sixty feet.
 * Shapechanger: The doppelganger is able to polymorph in and out of the form of a small or medium-sized humanoid it has seen before. With the exception of its size, its statistics remain the same, and none of its equipment is carried over between forms. As a doppelganger, it will revert to its true form upon death.
 * Ambusher: The doppelganger is at an advantage on attacks against creatures it has surprised.
 * Surprise attack: The doppelganger can deal extra damage upon successfully hitting a creature it has surprised.
 * Actions
 * Multiattack: The doppelganger can make two melee attacks.
 * Slam: A melee weapon attack that deals bludgeoning damage.
 * Read thoughts: The doppelganger can magically read the surface thoughts of one creature within sixty feet of it, and can continue to do so while the target is in range and its concentration is not broken. This effect can penetrate barriers, including up to three feet of wood or dirt, two feet of stone, or a thin sheet of lead.