BA tries to charge additional R4000

I booked myself a flight from CPT to London 12-29 June in Jan.<br> I purchased a 2nd seat on the same flight for my Life Partner but paid for this in Feb on a different card. <br> My life partner has just booked a VISA appointment for next Tues & said his e-ticket has my name on it. I contacted BA to find out why this was & to change it to his. BA said I couldn't change the name because it would be a \body swap". I asked how it could be a body swap if I am already on the flight. BA said the system does not recognise that there are 2 bookings with the same name. I asked them how this was possible. BA said that the flight I booked in Jan has my Avios card no."