Billing charges duplicated and missing

A billing pipeline consumes a Pub/Sub subscription and writes invoice line-items to Postgres. Finance reports two problems this week: (1) a few customers were *double-charged*, and (2) one batch of charges appears *missing*. Dashboards: the subscription's `ack_message_count` looks healthy, `oldest_unacked_message_age` is low. Recent context: the consumer was changed three days ago to ack the Pub/Sub message *first* and then write to Postgres (to 'reduce ack-deadline expirations'), and the Postgres write has no uniqueness constraint on `(invoice_id, line_item_id)`. There was a brief Postgres failover two days ago. Triage and explain both the duplicates and the loss.