What is the purpose of a freewheeling unit in an helicopter, if a centrifugal clutch is already present? Some helicopters such as the Bell 47 are fitted with a centrifugal clutch. 
Centrifugal clutch will engage only at certain engine RPM. If the engine fails, the engine RPM will reduce and the centrifugal clutch will disengage the rotor from the engine. So, why is an additional freewheeling unit needed?
the centrifugal clutch will only disengage after its rotating speed falls to the disengagement point. The over-running clutch disengages the engine from the rotor at the very instant that the engine's rpm falls below that of the rotor.

In this way, the rotor is never wasting kinetic energy by trying to turn over a dead engine and the vehicle stands a better chance of achieving successful autorotation.