SQL> create table test (id number);

Table created.

SQL> insert into test values (1);

1 row created.

SQL> insert into test values (1);

1 row created.

SQL> select * From test;

        ID
----------
         1
         1

SQL> alter table test add constraint uk_t_id unique (id) disable novalidate;

Table altered.

SQL> select index_name from user_indexes where table_name = 'TEST';

no rows selected

SQL> create index i1_t_id on test (id);

Index created.

SQL> alter table test enable novalidate constraint uk_t_id;

Table altered.

SQL> insert into test values (1);
insert into test values (1)
*
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.UK_T_ID) violated


SQL> insert into test values (2);

1 row created.

SQL> insert into test values (2);
insert into test values (2)
*
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.UK_T_ID) violated


SQL> select * From test;

        ID
----------
         1
         1
         2

SQL>