Those cards typically store the last few transactions (e.g. [0] is an Android app that can display transit card usage history), so it should be possible to code

Those cards typically store the last few transactions (e.g. [0] is an Android app that can display transit card usage history), so it should be possible to code idempotency into the card firmware. [0]: https://play.google.com/store/apps/details?id=com.codebutler.farebot https://play.google.com/store/apps/details?id=com.codebutler...