system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some helicopters have two rotors on top of one another while others do not?
gpt
The main rotor of a helicopter also causes the body of the helicopter to rotate in the opposite direction.  Normally, this force is countered by a rotor on the tail.  However, you can accomplish the same task by having another rotor on top that is rotating in the opposite direction.

At low speeds, two top rotors are a more efficient configuration.  However, they're less effective at high speeds.  So when you see multiple top rotors, it's normally on cargo helicopters or drones that don't expect to operate at high speed.