What would be the point? Contra-rotation has limited use as it is. In this case the stated reason is to ensure the axial flow has no energy wasted in twisting (

What would be the point? Contra-rotation has limited use as it is. In this case the stated reason is to ensure the axial flow has no energy wasted in twisting (though this can also be achieved with a stator) - in any case two stages are enough for this. The other main use is dissipating more power when you are up against some sort of limit on prop size, but this is not a common problem underwater.