Singapore, NTUC did the same to me. I bought 1 package of thai rice (5kg) with many other items. After I made payment, I noticed that I was charged for 2 packages of rice. I went back to the counter and reported. The supervisor corrected the receipt for me. After I reached home, I found out that, on the same receipt, I was charged for 2 packages of Yakult, which I bought only one...
Know the price before you pay.
Check the receipt before you sign (credit card).
Check the receipt before you leave cashier.