This is a quote from the book "On Zero Confirmation Transactions" by Chris Pacia
... this is fine, he has every right abstain from including those transactions. If he doesn’t include them, the next miner will. But the way it’s implemented is his pool doesn’t just abstain
from including them in blocks, but it deletes them from memory allowing double spends to take their place.
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So a new attack would look like this: Create two transactions; One sending
coins to the merchant with a second output sending a small amount of coins to a gambling service. Then create a normal double spend transaction. Send the first transaction to the merchant, walk...