Double-spend
A double-spend is the attempt to spend the same bitcoins twice — the core problem Bitcoin first solved for digital money without a central authority.
Before Bitcoin's invention, digital money without a middleman was impossible because files are trivial to copy. The double-spend problem asks: how do you ensure Alice can't send the same 1 BTC to both Bob and Charlie? Bitcoin solved this via the UTXO model plus Proof of Work: the first confirmed transaction wins, the second is rejected by every node.
Example
Alice tries to send 1 BTC to both Bob and Charlie. Both transactions hit the mempool; miners include one in the next block. The second becomes permanently invalid — her UTXO is already spent.
Frequently asked questions
Can I prevent a double-spend as receiver?
Wait for enough confirmations. One confirmation usually suffices for small amounts; 6+ for larger ones. More blocks on top = more irreversible.
Related terms
Further reading
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