When a digital currency is used fraudulently more than once before the transaction has been verified on a blockchain, it is referred to as double spending. In decentralized digital currencies like Bitcoin, where transactions must be verified by the network in order to avoid duplication, it is a serious issue.
Double spending is prevented by the consensus processes used in blockchain technology, such as Proof of Work (PoW), by validating transactions and ensuring their uniqueness before adding them to the distributed ledger.