A double spend is an attack where one or multiple coins are sent to two people or addresses at the same time.

This can be done by sending two transactions to different people or addresses at exactly the same time where coins that are to be sent exceed the number of coins present in a wallet.  Double spend resistance is built into Bitcoin and newer cryptocurrencies to prevent this occurrence.

Double spending has real life parallels where one cannot feasibly send more money than he has on hand in a cash only transaction.

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