Takealot refunds

I purchased merchandise through Takealot,#***0 and paid with PayFast through Capitec Bank an amount of R787. However I was under the impression that the payment did not go through, so I proceeded to pay the order with an ABSA Credit Card, then to my dismay the payment with PayFast also went through, so Takealot was paid two amounts of R787 instead of one. I have received the package, however Takealot is refusing to refund me the R787 which they were erroneously paid double for (I have proof of payment for both transactions). I realise that it was my own mistake to pay it twice, but does this mean they should just keep the second amount? I have tried negotiating with them telephonically but to no avail. I am very dissapointed in Takealot, you dare not make a mistake regarding payment, because you will not get it back, I am furious