What Is a Double Spend?
A double spend is a type of attack where the same digital currency is sent to two different recipients. Before Bitcoin, this was the fundamental problem that prevented the creation of a reliable peer-to-peer digital cash system. Bitcoin was designed specifically to solve this issue.
The Digital Dilemma
Because digital information can be easily copied, a malicious actor could try to broadcast two conflicting transactions at the same time, effectively spending their coins twice.
How Bitcoin Prevents It
All transactions are broadcast to the entire network. The first transaction to be included in a confirmed block is considered valid, while the other is rejected.
The Role of Confirmations
This is why waiting for confirmations is crucial. The more blocks that are built on top of a transaction, the more certain it is that it's the legitimate one and cannot be reversed.
A Fundamental Breakthrough
By using a public ledger (the blockchain) and a proof-of-work consensus mechanism, Bitcoin creates a system where everyone agrees on a single transaction history. This makes double spending practically impossible, providing the trust needed for a decentralized currency to function.