Outstanding Card Authorisations

Good day, can someone please tell me how the outstanding card amounts work? As soon as I swipe my card it says via sms that the money is off my account and gives me a new AVAILABLE amount, which of course I spend (Even on my internet banking it gives me the new available amount). But then later on the amount I swiped goes off again and then at the end of the month my account is overdrawn??? Does the money go off twice? Don't you guys put it in reserve? If I spend R1000 and my account says I have R500 available I am going to spend that R500. And then at the end of the month I am R1000 over my limit which then I lose on my next payday?? (And probably will affect my credit reputation) Please explain that to me, and I am sure I am not the only person that is worried about this. Like I said, if you tell me I have so much available after swiping my card and receiving my sms that the amount is off my account then its obvious that I am going to spend the available funds? I once had money deducted from account after buying goods at a shop 3 months earlier!!!