Most, but not all, relational DBMS products violate the very basic property of the relational model that a relation, whether base or derived, must not have duplicate rows. Why, and in what ways, does...

Most, but not all, relational DBMS products violate the very basic property of the relational model that a relation, whether base or derived, must not have duplicate rows. Why, and in what ways, does this violation give rise to serious problems?