This is a quote from the book "On Zero Confirmation Transactions" by Chris Pacia
...h the merchandise to broadcast the double spend, all miners will reject it.
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But here’s where things start to break down. While every miner is programmed to use this “first-seen�
�� policy, technically there is nothing forcing them to do so. In theory, a miner could patch their code to implement some other policy and in practice some have.
Consider the case
of the Eligius mining pool. The pool operator believes that certain types of transactions, such as gambling transactions, constitute “spam” and should be banned from the network. So he runs ...