Kulula selling the same seats twice

Having booked and paid for my flight, I pre-selected seats with extra legroom for both my outgoing and return flights which I paid for. On both flights however, I didn't get the seats I paid for as someone else was in my pre booked seat. Flying out was fine, they moved myself and my gf to extra legroom seats, but in my return flight I must sit in normal seats even though I paid for the extra legroom... I'm no lawyer, but that sounds like they're selling the same seat to two people and then the other must get stuffed! I believe that is *******? I want my money back for my return flight expenses... Crystal in the CT airport confirmed it wasn't a plane change either... This is unacceptable