How does Bitcoin prevent an output being spent twice?

Bitcoin transactions consume unspent outputs and create new ones. Each output can be spent only once in the valid chain. Full nodes validate the rules independently; accumulated work helps select among valid branches.

As a teaching analogy, exchanging old tickets for new ones differs from editing an account balance. The analogy illustrates linked outputs, not a claim that an output represents a physical ticket.

From broadcast to confirmations

A wallet broadcasts a signed transaction, but other nodes may first hold it among pending transactions. Inclusion in an accepted block gives it a first confirmation; later blocks extending that history add confirmations.

Nodes follow the valid history with the most accumulated proof of work. If the accepted branch changes, a transaction’s block can leave that history: this is a reorganization. More confirmations increase the work needed to replace the relevant history; they are not an unconditional guarantee. A receiver therefore defines when to accept a payment and how to handle a changed result.

Sources and further reading

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Bitcoin and ArcBlock

Bitcoin represents value as spendable outputs governed by spending conditions. ArcBlock Chain documents account records and typed asset operations. Compare how each authorizes an asset transfer before comparing application tools. See the chain-design comparison and ArcBlock transactions. This comparison does not imply a Bitcoin–ArcBlock bridge is available.