I have purchased 15 iPhones in Chennai and I want to bring them to Delhi. Will I have to pay duty for that in Chennai? I have the receipt proving I bought them in Chennai.
No, you don't have to pay duty fees when carrying cell phones from Chennai to Delhi. Both cities are in India, India is a unified federal republic, and import taxes are extremely rare between states. There are apparently some import procedures related to vehicles and some special goods into a small minority of states, but I don't believe this applies for bringing such a small number of cell phones to Delhi. Such laws are likely made for major retailers and big businesses. 15 is a relatively small number, bordering on personal use. They could all be gifts to friends and family.
I have crossed state borders in India many times, both through bus and through plane. I don't recall ever having seen any domestic customs checks. I don't even know how it would be possible to find facilities to try and declare something after a domestic flight or while crossing state borders by road.