Why does the Ka-50 (Hokum) have two main rotors, one on top of another?
The ka-50, like other coaxial rotor designs, use their contra-spinning rotors to cancel out the torque in the same way a traditional helicopter uses it's tail rotor.

A traditional single rotor:

Source

Contra-rotating rotors:

Source

There are a few benefits of the contra-rotating setup, such as


Reduced vibration
Reduced likelihood of settling with power
Reduced likelihood of anti-torque failure during flight.