How to avoid security breach at a subway/train station scenario?

I have a use case provided to me during an interview and I was given the following scenario to solve The question/scenario. Consider a metro subway or a bart station. There is a turnstile to enter which is done by scanning a magnetic card that has some prepaid credit. People are taking advantage of this by making... show more I have a use case provided to me during an interview and I was given the following scenario to solve

The question/scenario.

Consider a metro subway or a bart station. There is a turnstile to enter which is done by scanning a magnetic card that has some prepaid credit. People are taking advantage of this by making duplicate cards at home (buying card Readers/copying machines online) and copying the credit of 1 card into the newly created duplicate cards endlessly and getting into the train for free. How do you solve this issue?
I told them about using watermarks and having only original cards that would let you scan and enter. A followup question was thrown at me. What if they buy 2 original cards with watermarks with say $5 each and they just keep transferring the $5 from 1 card to the other (again by duplicating this at home). Thus, they enter with the original card which has unlimited balance. How do you solve this issue?
Thank you
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