A double spend happens when an attempt is made to send the same Bitcoin twice from one wallet to another, usually with nefarious intent.
Normally, the nodes verify that the Bitcoin in the wallet is available to spend by cross-referencing all previous transactions. The miners then process those transactions on to the blockchain. This proof-of-work mechanism is defined by Bitcoin’s protocol and is designed to prevent Bitcoin being spent twice.
However, a double spend attack isn’t impossible as if a malicious attacker is able to acquire over 51% of the hash rate, then they can manipulate the blockchain and send themselves more Bitcoin, especially if the transactions aren’t yet confirmed.
The likelihood of a double spend attack happening is relatively low as the amount of money required to buy up enough nodes, ASIC miners and the subsequent infrastructure to run a 51% attack would not make it worthwhile, given how many people there are verifying the chain today.
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