The double-spend problem occurs when a user tries to spend the same digital asset more than once. This is a potential issue in digital currencies because digital assets can be easily duplicated, unlike physical assets. In a blockchain, the double-spend problem is solved through the use of a distributed ledger and consensus mechanism. Each transaction is recorded on the ledger and verified by multiple nodes in the network using the consensus mechanism. This ensures that a digital asset can only be spent once because any attempt to spend it again would be rejected by the network.