I quite like the self-service tills, if I'm only buying a few items (basically, no more than two carrier bags worth) that won't require special authorisation and - critically - don't involve too many duplicates.
The barcode scanners on them seemed to be designed to be idiot-proof, so it appears to have a lengthy debounce timeout that hinders you scanning duplicate items. Best approach it to alternate them with items that have a different code, otherwise you stand there waving it about like an idiot while the scanner ignores you.
But I know people who simply can't get the things to behave at all. There's probably a quantum bogodynamics reason for this, but I'll give them the benefit of the doubt and blame Stan.