What is double spending?

Imagine paying for a coffee with a coin and, at the same moment, trying to hand the same coin to the newsstand. With physical money that’s impossible: the coin has already left your hand. A digital file, however, is easy to copy. So how do we make sure digital money isn’t spent over and over again?

With bank payments, the bank checks the balance and records the transactions. On a decentralized network such as Bitcoin, this job is done through shared rules, checks by the nodes and a consensus mechanism, so that the network agrees on which transaction history is valid.

For example, someone sends the same bitcoin to a merchant and to an address of their own in two conflicting transactions. Bitcoin’s rules don’t allow both to remain valid in the same history. A digital signature proves that a payment was authorized, not that the money hasn’t already been spent. The history has to be checked too.

Why it matters

If you accept crypto as payment, a transaction showing up in your wallet doesn’t necessarily mean it has been confirmed. On Bitcoin, one confirmation means the transaction has been included in a block. The more blocks are added on top of it, the harder it becomes to rewrite its history.

We consider the distinction between “I saw the payment” and “the payment has enough confirmations” crucial. There’s no single number of confirmations that suits every network and every amount. Finality rules and risks vary.

What to watch out for

A chain reorganization can replace recent blocks and drop a previously confirmed payment. On Proof of Work networks, a 51% attack can be used for such a reversal and to double-spend the attacker’s own coins. It doesn’t automatically give the attacker access to other people’s private keys.

If you’re expecting a payment, check the transaction on a block explorer, not just in a screenshot from the sender. A “pending” status isn’t a confirmation, and a delay on its own doesn’t prove a double-spend attempt.