It shouldn't be too hard a problem to solve - ethernet has managed to do
collision detection since the 80s. I guess it depends on how smart they
want to make the hardware in the card.

--
Spud

My Oyster card will not read if I have a PATH Smartlink card next to it.
However, at least four years ago when I last used it, the Smartlink card
will work quite happily on the readers on the PATH turnstiles when next to
an Oyster card, so it can't be too difficult to make it ignore a 'foreign'
card not valid on that system. Of course it's slightly more difficult when
two valid cards, say an Oyster and a VISA debit card can both be seen.