How oracle allowing duplicate values for unique indexes which already defined for a table object ?

I have tables with unique indexes(composite of  columns), though it is unique indexes attached to the table object, still
 having duplicate values for the same unquie index key values.

SQL> create table dummy_objects (object_id  number, object_owner varchar2(30), object_name varchar2(30));

Table created.

SQL> insert into dummy_objects values (0,'MIHIR','TABLE_A');

1 row created.

SQL> insert into dummy_objects values (1,'HAPPY','TABLE_B');

1 row created.

SQL> insert into dummy_objects values (NULL,'BMK','NULL_1');

1 row created.

SQL> insert into dummy_objects values (NULL,'JOHN','NULL_2');

1 row created.

SQL> create unique index dummy_objects_u1 on dummy_objects(object_id);

Index created.

SQL> drop index dummy_objects_u1;

Index dropped.

SQL> create unique index dummy_objects_u2 on dummy_objects(object_id, object_owner);
create unique index dummy_objects_u2 on dummy_objects(object_id, object_owner)
                                        *
ERROR at line 1:
ORA-01452: cannot CREATE UNIQUE INDEX; duplicate keys found