Double-Spending is the act of using the same bitcoins twice. There is only a 21 million set cap on the protocol and no more can be produced. So the network protects against double spend by the verification of each recorded transaction. The blockchains ledger ensures that the transactions are finalized by its inputs confirmed by miners. The confirmations make each unique Bitcoin and its subsequent transactions legitimate. If one tried to duplicate a transaction the original blocks deterministic functions would change showing the network that it is counterfeit and would not to be accepted.