Double-Spending

Double-spending is the act of using the same unit of digital currency more than once. Unlike physical cash, which must be handed over, digital data can be copied. If a network cannot verify that a coin has already been spent, the currency's integrity collapses. Traditional banks solve this with a central database that logs every debit and credit.

Decentralized networks have no central authority, so they use consensus algorithms instead. Proof-of-work and proof-of-stake require participants to invest computational effort or stake assets, making it prohibitively expensive to rewrite transaction history. Each transaction is checked against the full ledger before acceptance. Once confirmed, the token is marked as spent and cannot be reused.

Preventing double-spending is the foundational problem that Bitcoin solved and the reason blockchain exists. Without it, decentralized digital currency is impossible.