What Is Double Spending? | Cryptocurrency Glossary

Measures to Prevent Double Spending in Bitcoin

Double spending occurs when Bitcoin that has already been used for a payment is used again for another transaction. For
example, it refers to a situation where a user, despite owning only 1 BTC, pays 1 BTC to Person A and then pays another 1 BTC to Person B.

In Bitcoin, transactions cannot be executed without a signature from the user who holds the private key—which only the owner possesses. Since transactions are conducted based on the trust that the payment comes from the legitimate owner after verifying this signature, fraudulent transactions typically do not occur. However, it is possible for the true owner, who holds the private key, to make a double payment. Of course, this is not limited to intentional fraudulent acts; it is also possible to accidentally use the same coin twice.

Therefore, Bitcoin has implemented a mechanism to prevent double spending by incorporating a confirmation process on the blockchain. This system stores all records of past transactions and verifies their legitimacy.
However, if an orphan block is created, there is a risk of double spending occurring in the main blockchain.

For this reason, it is recommended to verify that a transaction has received at least six confirmations before trusting it. In the
unlikely event that you conduct a transaction using a Bitcoin that has been double-spent, all subsequent transactions may be invalidated and reversed later. To avoid getting caught up in such trouble, it is safest to use Bitcoin that has received six confirmations.