52 GREEN ROOM. The door to this room is painted green. The walls and floor are painted jade and dark gray-green in a checkerboard pattern. A freestanding variant of the mirror of opposition is bolted to the center of the room.
A green-skinned duplicate will appear for each person who states that they look in the mirror. It will take several seconds for the materialize from green smoke, so it is possible for several characters to look before the first duplicate appears.
Each duplicate will have the hit points and armor class of the original, strike once per round with a normal version of the original's weapon, and attack as a 4 hit die monster. They will not have the special abilities (spell-casting, backstab, etc.) of the original but are 50% magic-resistant. Each is worth 125 points of experience plus an additional 4 per hit point.
Once the last (or only) duplicate is defeated, it will disappear in a cloud of green smoke, leaving behind a green key. Together with the red, yellow, and blue keys it will open the green door to Room 53.