This is a quote from the book "On Zero Confirmation Transactions" by Chris Pacia

Well not so fast

While we can indeed double spend unconfirmed transactions with a very high success rate, the merchant should be able to trivially detect mos
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...
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