Outrageous service from Doha head office

On the 1st of April I was in the UK and booked on line, with Qatar Airways a single ticket from Malaga, Spain to Durban for the 25th April. They asked me for a payment (Credit card) verification and after submitting copies of passport, ID, credit card statement and even police clearance, they confirmed my booking with the condition that I present my card at the airport. I did exactly that when I checked in for my first leg in Malaga for the flight to Barcelona. Then I said goodbye to family and flew to Barcelona, where the airline asked me to present the Amex card again. They then proceeded to show me my card was black listed I could not get on the flight without paying again. I was alone, distraught, stressed, angry and in tears. I had presented to the staff all my original documents, but it was the head office in Doha calling the shots and I was told I had to pay again to get on the plane. I had no option, I paid again, this time with my master card which, is in exactly the same name and comes off the same account, as the Amex. So why the double payment? It was also not a cheap single ticket and now they have 2 payments from me, refusing to refund. It is unacceptable and very sad that their head office can ruin the reputation, when the airport and on-board international staff are very helpful. Could the locals learn from this? I can Ill afford this extra expense and will continue my fight for my refund, whatever it takes.