Database tables defined with the SET attribute in the CREATE TABLE kind clause do not permit duplicate rows. Any time a row is inserted into a SET table having a NUPI and no uniqueness constraints or indexes, the system performs a duplicate row check.
When Vantage checks for duplicate rows, it considers nulls to be equal, while in SQL conditions, null comparisons evaluate to UNKNOWN. See Inconsistencies in How SQL Treats Nulls for information about how nulls are interpreted by commercially available relational database management systems.