doublespend

Double spend is the attempt to spend the same digital currency unit more than once. In digital asset systems, coins exist as entries in a ledger rather than physical objects, and the validity of transactions depends on updating balances in a single, authoritative record. If a user initiates two transactions that draw on the same funds, only one can ultimately be accepted as valid.

In centralized systems, a trusted authority maintains the ledger and prevents double spending by updating balances in real time and settling transactions before funds are considered spent. In decentralized or blockchain-based systems, prevention relies on consensus and a shared history of transactions. For UTXO-based networks, a transaction consumes an output, and subsequent transactions cannot reuse that same output once it is spent in a confirmed block.

Common double-spend scenarios include race attacks, where two conflicting transactions are broadcast and one is confirmed first, and more severe attempts such as 51% attacks, where an entity controlling a majority of the network’s resources attempts to reorganize the chain to invalidate prior confirmations. The risk of a successful double spend decreases as additional confirmations are accumulated.

Mitigation strategies include waiting for multiple confirmations before finalizing a payment, using payment channels or escrow for instant transactions, and employing finality mechanisms in consensus protocols that reduce the chance of chain reorganizations. Merchants and wallets may employ additional checks for suspicious activity or rely on trusted intermediaries for high-value transfers.