Nando's is a ****

So Today, I used Nando's voucher that I got from Vodacom, I app**** the same voucher multiple times on my Nando's app and it went through even on my checkout, but when it was time for me to collect my order I was told that I need to pay for my order, which I didn't understand why, because I settled my order using a voucher. I ended up not getting my order because they insisted I pay whilst I paid for my order using voucher. If I wasn't supposed to use the same voucher multiple times , their app shouldn't have allowed me or it should have stated that the voucher was already redeem, so their app fooled me and allowed the same voucher to be app**** multiple times.. I should have received my order, because that wasn't my problem but theirs with their **** system that allowed me to apply same voucher to settle my order.. I'm so disappointed with Nando's