If you make bitcoin transactions regularly, you may notice the option "Replace by fee".
Replace by fee (RBF) means that the sender can resend the transaction with a higher fee, if they accidentally set a low fee and don't want to wait a long time for confirmations. Miners then see the higher fee transaction and are more likely to include your tx into the next block.
The unconfirmed low fee transaction is now invalid and is ignored, because the bitcoins were already spent in the confirmed high fee transaction.
What about double spending?
RBF is a method to accelerate a slow transaction by using a higher fee. It isn't intended for double spending and it is possible to double spend an unconfirmed transaction without RBF flag.
Double spending doesn't generate more bitcoins. The same bitcoin appears to be sent to 2 people, but only 1 person receives the money (the confirmed higher fee tx) and the second person receives nothing (the ignored lower fee tx).
In addition to double spend attacks, unconfirmed transactions can be stuck, orphaned or removed from mempool.
This is why sellers and traders must always wait for at least 1 confirmation for a safe transaction. Once a transaction has 1 or more confirmations (takes on average 10-20 minutes) it is safe from double spend attacks. For high amounts traders may prefer to wait for 3-6 confirmations for more security.
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