Urgent - GST on house construction by builder for self

It is works contract.

All the bills received after July, 2017 will carry 18% GST.

But this work contract includes material also which has to be bought by him for giving possession of house in ready to move condition.

As he was supposed to pay GST on all the material bought for construction. which obviously he will be paying from my money (fixed 1500 ₹ / sqft) .
if I pay 18% on contact amount too, I will be paying GST twice for one purpose.

whereas, GST can be imposed only once for one purpose.