Charged twice for one item

Bought a box of Brown Onion powder @ Checkers Hyper Festival Mall the cashier scanned for 1 item and charged me for two , when I brought this to managements attention I was advised that it is a promotional item , my problem wasn't about the item being on promotion or not my problem is that I had one item and they charged me for two after standing and arguing with management for almost have an hour wasting my valuable time , they decide to give me two of the same items if I wanted two I would have bought two, I will just stick to Pick and Pay......