This is a quote from the book "On Zero Confirmation Transactions" by Chris Pacia
...t double spend attempts as they happen. Last year Gavin Andresen and Tom Harding wrote some patches for Bitcoin Core that would have relayed the first double spend attempt for each transaction a
round the network (double spends were not previously relayed and would not reliably propagate the network). With double spend relaying a retail point-of-sale terminal could have detected a doubl
e spend attempt and flagged the cashier to halt the transaction. The screen might read ‘payment declined’ and the customer asked for another form of payment. Should the original transaction ...
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