Double-spending is the risk of using the same digital currency unit more than once. Unlike physical cash, which cannot be duplicated, digital currency can potentially be copied. This creates a concern that someone could spend the same coins in multiple transactions. To prevent this, most cryptocurrencies use a decentralized ledger called blockchain. Each transaction is recorded in a block and securely linked to previous blocks, making it extremely difficult to alter past records. This setup ensures that when a transaction is confirmed, the coins used in that transaction cannot be spent again.Additionally, mining or validation processes help verify transactions, ensuring they are legitimate before being added to the ledger. The consensus mechanism, like Proof of Work or Proof of Stake, further strengthens this verification, making it nearly impossible for someone to manipulate the system without vast resources. Overall, double-spending is a significant challenge that systems are designed to combat, ensuring the integrity and reliability of digital currencies.