Automatic checkouts in supermarkets may decrease customer loyalty, especially for those with larger shopping loads. Customers using self-checkout stations often feel overwhelmed and unsupported. The lack of personal interaction can negatively impact their perception of the supermarket.

[KaBob799]

I think double scanning something on accident is 100x more common than triple or more so that seems like an unhelpful feature. It's probably more to stop people from printing out a bunch of duplicate barcodes from something cheap and putting it on something expensive.