Card transactions deducted double

when I bought things on my card it went of from the available balance and when the transaction is cleared on my statement it deducts again.

NEDBANK how is is possible if I dont have an overdraft that my account can go into a minus value because of efts and card transactions. and I dont mean late entered transactions like bakwena where the card is swiped but the order is only sent to the bank on a later stage.

I would like to Know how your book system works.