Refund on debit order

I took some money from a paid up FNB bond account transferred into FNB cheque on 20th Feb and on the 24th Feb I returned all the money through eletronic transfer again from current to bond account within the same bank. The 26 they debited my account the same amount I had paid. But because I had paid, there were no funds for that debit order to go through. They charged me for that yet I had already settled before their Debit order. I have been to the branch and several emails to know avail. I cannot understand why the cannot refund me my R100 for insufficient funds on a attempted debt order I had already settled.