If it was exported out of Canada and imported into US , then it is considered a US vehicle. When you want the car back into Canada, you will have to follow the same instructions as here:
Registrar of Imported Vehicles
Now, there will be GST on the vehicle plus other fees. BUT in regards to duty, that depends on the heading of the VIN number, if it is a number then the car falls into the NAFTA agreement for duty free. Which will save you 6.1%.