Double spending is the risk that a digital currency could be spent twice — the fundamental problem that all cryptocurrency consensus mechanisms are designed to prevent. Unlike physical cash, digital data can theoretically be copied, so blockchain networks need a reliable way to determine which transaction is the "real" one.
How Blockchain Prevents Double Spending
Blockchain solves double spending through its consensus mechanism. In Bitcoin's case, miners validate transactions and add them to blocks. Once a transaction is confirmed in a block (and subsequent blocks are built on top), it becomes extremely difficult to reverse.
The Confirmation Rule
This is why merchants and exchanges wait for multiple confirmations before considering a transaction final. Bitcoin typically recommends 6 confirmations (about 60 minutes). Each additional confirmation exponentially increases the cost of attempting a double-spend attack.
Vulnerabilities
A 51% attack could theoretically enable double spending by allowing an attacker to create an alternative chain that replaces recent transactions. This is why smaller, less-secured blockchains are more vulnerable to double-spend attacks.