Jet Airways India - Ticket refund

I changed a flight from Delhi-Mumbai on Dec * to Dec *. When I made the change online, I was assured by the Jet agent that I would get a refund. When I didn't I wrote to Jet and was informed by them that I had used both tickets so was not entitled to a refund. This is a false statement as the original ticket was for Dec * at *.** am and I actually flew on Dec * at * pm. If I had used both tickets, would my name not be on a passenger manifest, and a record of me actually having flown? Am I to be concerned that their security measures are now so lax that someone else flew in my place? Or is this an outright lie? Thank you.