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I'm reading through the Mastering Bitcoin book and the section talking about what nodes check for to ensure a transaction is valid before accepting it refers to this:

Each node verifies every transaction against a long checklist of criteria:

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For each input, the referenced output must exist and cannot already be spent.

However, I'm not sure exactly what it means for "an output to exist". How can an output exist if I am creating it now with this transaction?

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After reviewing the structure of a transaction, I now understand what is meant by that statement.

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The input is the UTXO, and the output is the output of that UTXO. So when it says the output must exist and cannot already be spent, it's saying the output of the UTXO must exist and not already be spent.

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It says "referenced output", which means output of previous transaction. (The one you are spending.)

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