Any advantage of a counter rotating outdrive on a single. Looking at a boat and it has a coutner rotating lower unit. Would it help counter react the engine torque ?

Some boats use left hand rotation props (Fountain 27' & 29's for instance) for a variety of reasons. Using a counter rotating drive in a single is very unusual. For one thing, it's going to make finding a prop a lot more difficult. Is it original to the boat or did somebody score a cheap drive because it was avaialble?

Yes. Counteracting torque is exactly what it is meant to do. The extent to which it helps depends on the hull. When i put bigger power in my velocity, i changed to counter rotating and it made a small improvement at high speeds.

Are you referring to duo prop drives?

I don't see why it would make finding a prop more difficult. Everyone that makes a RH prop makes a LH, and RHs are more desirable as that's what the majority of singles run. There's tons of LH wheels out there that are left over mates to scavenged RH prop users.