How does Bitcoin solve the double-spending problem?
Correct answer
Answer A: Nodes check whether the amount has already been spent.
Quick explanation
Bitcoin participants follow shared rules to check whether an amount can still be spent. Conflicting attempts to spend it therefore cannot both be valid.
Detailed explanation
With cash, the same banknote cannot be handed to two different merchants at once. Digital data, however, can be copied. An attacker might therefore try to broadcast two conflicting transactions spending the same available bitcoin amount. This is called double spending.
Centralized payment systems solve it through an operator maintaining an authoritative database and deciding which entry was valid first. Bitcoin replaces this central decision with digital signatures, network relay, independent rule checking and a block history ordered through proof of work. Nodes do not accept a transaction attempting to spend already-spent value again. Confirmed blocks establish a shared order that participants can follow.
This does not mean nobody can create or send two conflicting transactions. What matters is that the network does not treat both as a valid confirmed history. Unconfirmed payments therefore still carry risk. Additional confirmations make later changes increasingly costly. The detailed implementation is explored in the transaction and mining topics.
Example or everyday application
A digital event ticket can be copied endlessly as an image. The entry system must still ensure that only one use counts as valid. Bitcoin needs comparable uniqueness without a single organizer keeping the central list.
Common misconception
A beginner might think copying a wallet file creates new bitcoin. Only data or keys are copied; the network does not thereby accept an additional valid spend.
Sources used
Bitcoin Developer Guide – Transactions
This source is mainly intended for developers. The relevant information may therefore be harder to find.
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