RFID scanning in supermarkets

Hopefully someone can clear this up for me; at a supermarket checkout with barcodes it's easy to ensure you only scan things once but with RFID it detects tags within a field of space so what's to stop it scanning things twice?, or accidentally scanning the tin of carrots the old lady behind you has in her basket?.
Radio waves are very difficult to control so I don't see how you can be sure they're limited correctly, if a pringles tin can be used to create a long distance wifi antenna then who's to say that in the right circumstances it wont extend the range of your checkouts scanning signal?.
I don't see what's wrong with barcodes in that situation that creates the need for RFID, if you need to store more info in the barcode then just use a datamatrix - http://en.wikipedia.org/wiki/Datamatrix