Dataflow "mark as a key" VSwithout key column

When a dataflow output does not have a key column marked, Power BI does not perform any form of update or upsert; instead, it simply treats each row as a new record and inserts it into the destination table (such as a Lakehouse or Warehouse). This means that even if a record with the same values already exists in the destination, the dataflow will still insert the new row, resulting in duplicate entries over time.

Since there’s no key to match on, Power BI has no way of identifying whether a record should be updated or replaced. Therefore, if your data includes updated values for existing records and you want those to overwrite or update the previous versions, you must mark a key column to enable upsert behavior. Without a key, the dataflow acts purely as an append-only operation.