Where a wholesale dealer's licence for the sale of any liquor is taken out by a person who is the holder of a licence authorising him to sell the same liquor by retail, the duty payable on the wholesale dealer's licence shall be reduced by fifty per cent. Provided that this provision shall not operate so as to reduce the total amount payable for the wholesale dealer's licence and the retailer's licence for auy liquor below that payable for the wholesale dealer's licence alone.