Function find_duplicate_position 

pub fn find_duplicate_position<K>(
    keys: impl IntoIterator<Item = K>,
) -> Option<(usize, usize)>
where
    K: Eq + Hash,

Detect duplicate keys in a batch input, loud-failing the whole request when found. Returns the first duplicate (by position) so the surfaced error can name a specific offending index. Linear- time, allocation-only in proportion to the input length.

Used by all five batch primitives — batch_get, batch_delete, batch_update, batch_create (when the input carries a client- supplied PK), and batch_upsert. The dedup posture is deliberate: silently collapsing duplicates would break the per-item index mapping the envelope promises and hide caller bugs.