doublespending

Double-spending, also written as double-spend, is the risk that the same digital unit of value could be spent more than once. In physical cash, handing over a note prevents a second payment. In digital systems, where copies can be made easily, preventing this requires an authoritative ledger or a consensus mechanism that ties each unit of value to a single, final transaction.

In centralized systems, a single database or trusted administrator can guarantee that a payment recorded there cannot be spent again. In decentralized cryptographic networks, distributed ledgers and consensus protocols (for example proof-of-work or proof-of-stake) attempt to make double-spending unlikely by making conflicting transactions progressively harder to accept.

Common risks include zero-confirmation (0-conf) transactions, which are accepted before any block confirmation. A race attack can spend the same funds in two conflicting transactions if a merchant accepts an unconfirmed payment. A more severe threat is a majority (51%) attack, where an attacker controls enough hashing power to reorganize the ledger and invalidate prior transactions.

Preventive measures include requiring one or more confirmations before finalizing a payment, delaying settlement for high-value transfers, and implementing risk assessment in merchants' payment flows. Other approaches include payment channels or layer-2 solutions that settle off-chain and then commit to the main ledger. Good network security and up-to-date software also reduce the risk of exploitation.

Double-spending remains a central concern in digital cash systems; most modern networks rely on probabilistic finality and economic incentives to deter it. While the threat is real, robust implementation substantially reduces its likelihood.