I've book an Air Deccan round trip flight with you, among other services, I would like to know if I'm allowed to get the inbound flight (from Amritsar to Delhi) if I do not catch the previous outbound flight from Delhi to Amritsar. I mean, if I do not catch the flight from Delhi may I come back to Delhi or the tickets are automatically cancelled?

Most airlines will cancel the next leg of your roundtrip based on no shows - unless you show up at the airline ticket desk or call the airline and get your ticket endorsed.

In most domestic Indian flights, including Air Deccan, most reservations are based on one-way travel. They do not require round trip. However, you still need to contact the airline to let them know that you are not going on the outbound.