Do I get a prorated credit when purchasing the set since I already own the driver?
Don't think so.
Will I end up with two identical drivers in my inventory?
This is difficult by definition, as I think you should not be able to buy the same club twice(?) but I've no idea what happens if you walk into a conflict, buying the set.
Did you have a look if the set is released for purchase for you?
It might be the safest to sell the driver before buying the set.