FNB double debits

In April I used my debit card to pay for my flight from Mozambique to South Africa. I was charged twice on my account, one debit order take place on the 20th of April the other on the 8th of May. I've been going between Saa and FNB so many times but no one wants to take responsibility for it. I fail to understand how FNB authorised two payments of the same reference. On the other hand Saa says they did not bank in the 2nd payment as it doesn't reflect on their accounts. I'm now currently sitting at a -R4000 in my account because of this even the money I had in there its gone. It's clear to me that someone is lying here between FNB and Saa.