Function zebra_state::service::check::nullifier::tx_no_duplicates_in_chain

pub(crate) fn tx_no_duplicates_in_chain(
    finalized_chain: &ZebraDb,
    non_finalized_chain: Option<&Arc<Chain>>,
    transaction: &Arc<Transaction>
) -> Result<(), ValidateContextError>

Reject double-spends of nullifiers:

• one from this Transaction, and the other already committed to the provided non-finalized Chain or ZebraDb.

Consensus

A nullifier MUST NOT repeat either within a transaction, or across transactions in a valid blockchain. Sprout and Sapling and Orchard nullifiers are considered disjoint, even if they have the same bit pattern.

https://zips.z.cash/protocol/protocol.pdf#nullifierset