An attempt to spend the same token or digital asset more than once. It is similar to trying to pay with the same banknote in different places in the real world. In the digital world, however, assets can be copied, so the system has to make sure that the same asset is not spent twice. To prevent double-spending, blockchains use consensus mechanisms, transaction confirmations, balance checks, and, in some cases, the tracking of unspent transaction outputs (UTXOs).