dbt incremental unique_key silently dedupes during backfill

dbt incremental materialization with a unique_key issues a MERGE on that key. During a backfill where multiple source rows share the same unique_key across days, only ONE row survives — not the latest-per-day as intuition suggests. The surviving row is backend-dependent: BigQuery dedupes with undefined order, Snowflake by row-scan order. Fix: use a compound key like ['id', 'event_date'], OR switch to incremental_strategy='insert_overwrite' with partition_by when the table is date-partitioned — that mode replaces whole partitions instead of merging on keys.