Function check_effect_exactly_once 

pub fn check_effect_exactly_once(journal: &Journal) -> CheckResult

effect-exactly-once — the crash-replay double-side-effect bug, by construction: effect_id names an intended-once effect (a deliberate re-execution is a new id), so the same id performed twice is a defect — and the evidence says whether it was replayed across a resume boundary (the classic durability bug) or duplicated within one live run.