FNB has accounts with the same 11 digit account number within Namibia and SA

A payment was made via Nedbank to a FNB Namibia Customer. The sort code used was the General sort code rather than the Namibian one. As one can pay ZAR to Namibia it was assumed that the general sort code would work. The payment was made and it was found out later from the supplier that they never received payment. Eventually it was determined that the money had been paid into a SA FNB Account with exactly the same account number. There are two issues. Within FNB why are there two exact same account numbers? The probability of two accounts existing with the same 11 digit account number is as close to zero as is possible. I bank with FNB and my Private Banker was a helpful as possible but it was explained to him that FNB really can have two 11 digit account numbers in SA and Namibia.