Thinking about this more, I'm surprised this couldn't be caught when the items got delivered to Amazon. I find it extremely unlikely that a picking robot accidentally found an item with the same barcode if they weren't already in the wrong place.
Seems like when the items arrived at the fulfilment centre, they got mis-scanned and ended up comingled. Presumably that's the stage where you can check weight and volume - eg does this item fit with the known dimensions.
This check must be made somewhere otherwise people wouldn't bother returning high value electronics with rocks inside (presumably someone does a cursory check of weight before the inventory gets comingled again).
It's not unusual for warehouses to deal with items with multiple barcodes.
For a technology example, you might get a network card with a UPC barcode, but also a MAC address barcode, and a manufacturer's part number barcode. A wholesaler/ manufacturer sells boxes of 20 items to resellers? The outer box will have a barcode. That box got sent by courier? Three barcodes on some mailing labels.
So at a goods-in station, the usual response to "multiple barcodes, some don't make sense" is "Keep trying until you find one that does make sense"
Those multiple barcodes also confuse the crap out of self-check-out stations. Once it picks up on the wrong one, it throws a fault that the attendant has to come back out of, so you can't just back out scan the right one.
What baffles me is that the store/software can't build a dictionary of "known misdirection" barcodes, like "this is the shipping carton not the product itself, fault and tell the user to rescan, but don't just lock up" when they're seen.
Scanners in distribution centers contain a barcode hierarchy that filter out unused barcode types. For example, after something has been picked, a UPS label might be applied to it. Subsequent scanners will be programmed to only read either the internal routing label or the UPS label (or both and let the software handle it).
The scanners in supermarkets are made by the same companies and I would assume run on the same software. However, it might be more difficult to ensure a specific type of barcode is used on 100% of products in the store. So if you see the scanner picking up the wrong barcode at the check-out aisle, it is most likely either the scanner is not programmed correctly, or the scanned barcode is the same type as a valid barcode used somewhere else in the building.
As far as handling faults, that would probably be done on the POS system, not the scanner itself. The scanner software is perfectly capable of handling errors in different ways (for example, sending a specific code to indicate two different non-matching barcodes were read in the same pass), but from a functional standpoint, the scanner is 'dumb'.
Seems like when the items arrived at the fulfilment centre, they got mis-scanned and ended up comingled. Presumably that's the stage where you can check weight and volume - eg does this item fit with the known dimensions.
This check must be made somewhere otherwise people wouldn't bother returning high value electronics with rocks inside (presumably someone does a cursory check of weight before the inventory gets comingled again).