Volvo Penta DuoProps have a front and rear propeller that rotate back to back on the same drive. The front propeller is closest to the boat and hits the water first.
DuoProps have both a counter and standard rotation prop to eliminate steering torque. The front prop is always counter rotation. The rear prop is always standard rotation.
Volvo Penta DuoProps are each stamped with a size code (D2, F5, T9, etc) that is used to find a prop replacement. With this code you can replace either the front or rear pieces, or the complete DuoProp.