VAT on discounted goods @ brand fractory

I bought cloths from Brand Factory and the MRP says including of all taxes, they have discount of 50% which they devided by 50% and added VAT on this @5.5%, the manager said that any sale over 20% discount will be added with VAT, as the MRP is inclusive of taxes they should reduce the taxes and then devide 50% then add the VAT,
This is double taxation for the same good which is illegal

Please advice how to file consumer court complaint against this fraud