ORA-01442: column to be modified to NOT NULL is already NOT NULL

create table tq84_t (n number);
-- Table TQ84_T created.

alter table tq84_t modify n not null;
-- Table TQ84_T altered.

alter table tq84_t modify n not null;
-- ORA-01442: column to be modified to NOT NULL is already NOT NULL

drop table tq84_t;
Table TQ84_T dropped.