Your card can be declined on a cashier’s screen and holding your money at the same time. Both are true at once, and that is why the second swipe is the one that costs you.

When a card is declined, the bank doesn’t just say no. It returns a two digit code explaining why, and that code decides what should happen next. Code 91 is a communications failure, and running the card again is correct. Code 51 is insufficient funds, and payment processors tell their own merchants in writing that retrying a 51 without authorization can expose them to a chargeback for invalid authorization. You never see the code. The cashier does.

This one covers what the card reader and your bank actually do in that second and a half, why a banking app cannot answer the question you are asking it at the counter, the exact words to use before the card moves again, and what to do if you have already been charged twice. Visa reason code 12.6.1, duplicate processing, and roughly 120 days to file it.