That's a problem sitting on the barrier-side of the mechanic. Currently, ticket barriers read the first NFC they get aware of, test it if it's allowed, let you through if yes, reject you if not. The barriers aren't even checking if the chip they read belongs to them. Reading all chips the reader can see is possible, determining which one(s) can theoretically contain valid tickets is easy.I'd say "roll on smartcards", but unfortunately, everything seems to have moved to interfering smartcard technlogies now, meaning that you end up having to take the individual smartcard out of the wallet for it to work, as it interferes with your Oyster, office access card, and 3 NFC credit/debit cards.