In almost a decade of shopping there this has never happened to me and I'm one of those bastards that reads the receipt as I'm boxing the groceries before I leave.

I joked with a cashier about this, once ... she said, deadpan, "it beeps when it scans". I'm guessing they're listening for two beeps when they're expecting one? The store keeps stats on the cashier's times so I'd imagine there's some training and probably some things only an Aldi cashier could speak to about how they manage to be quick, accurate and deal with the 30 or so customers a day who (used to) come in expecting to pay with a credit card (they're now accepted at Aldi in the US).

Given the way that barcode scanners work, as the sibling comment points out, there is almost certainly some kind of 'debouncing' going on. The system is acquiring data fast enough that it probably has enough time to repeatedly read the code tens or even hundreds of times before you move it out of the field of view. So I'd guess it triggers on the first detection and you get a short dead time to move the product away.

This is exactly how it typically works. There's a time that needs to pass before the scanner accepts the same barcode again, and any other barcode scanned resets the lock for the previous barcode (so you could scan two different items over and over in very quick succession, but this doesn't work with just one item). The time is configurable on the scanner and typically in the 1-2 second range.

As I work in cash register software development and sometimes wanted to perform performance tests on real hardware and without mocking the scanner away I have developed a few tricks around this rescan prevention delay. One involves a wheel with different barcodes printed on the side that is attached to a battery-powered screwdriver machine. The more sophisticated one is a belt made of paper with different barcodes on it that is always being rotated around using wheels in a little machine built using LEGO Technic parts. It is pretty awesome how mind-boggingly fast those bigger laser barcode scanners can scan barcodes when they are presented to them by the LEGO scan robot :D

I've never heard that happening. Usually those systems prevent scanning the same barcode in short succession and cashiers have to manually enter the amount if more than one is bought.

I've seen a cashier sometimes scan something twice but I think it was human error of some sort.

She just pressed a button on the register and continued. I think she used the same functionality for scanning one item while you really bought 12 of them.