Paying back for a mistake made by the bank

So Fnb rewarded me with an ebucks voucher but their system messed up and gave me more vouchers. i called in and was advised that what ever vouchers are under the voucher tab belongs to me. i called in and let FNB know that their is system glitch and i only found this out after i had used the vouchers. now for being honest the team leader at FNB ebucks advises me i need to pay it back a if this was my fault. i think this is unfair and the manager needs to get involved.