If the distribution key column in a target table contains null values and you configure a task with an

Update as Insert

operation for the same target table, the task might create duplicate rows. To avoid creating duplicate rows, you must perform one of the following tasks:

Replace the null value with a non-null value when you load data.
Do not configure the column as a distribution key if you expect null values in the distribution key column.
Remove the distribution key column from the target table temporarily when you load data. You can use the Pre-SQL and Post-SQL properties to remove and then add the distribution key column in the target table.