There are two shops, at the opposite ends of the arrivals hall. Same operator, but the available selection could be different at times. I have used them; prices and convenience are attractive. Prices are in dollars, but they do take rupees (not sure if citizenship affects that). You have to produce the passport; this may be to prevent airport employees from duty free buying/reselling business.

Please note that, like everywhere else, India too doesn't allow liquor in carry-on baggage. Therefore I don't buy liquids if connecting to a domestic flight, only for gifting in Delhi or when connecting via train/road.