Idempotency means:
> Running the same operation multiple times produces the same final result.
If a load of 1000 records fails halfway and Airflow retries with a blind INSERT, you may end up with 2000 rows.
Safer patterns:
MERGE / UPSERT DELETE + INSERT for a partition Replace partition Unique / natural keys
Interview tip: Airflow retries are only safe if tasks are idempotent (or compensated).