Whilst unfortunate, it isn't common. Was it a Spitfire and P-51 by any chance? Is there a third kit listed on any paperwork in the postage box? I’d bet it should have been a Mig-17. If you check the thread: “What was in your mystery box”, another user had the same thing happen. I would email customer services and they will hopefully either send it out or perhaps send you a partial refund.