SQL> create table a1_test (id number primary key);

SQL> create index i1 on a1_test (id);
create index i1 on a1_test (id)
                            *
ERROR at line 1:
ORA-01408: such column list already indexed

SQL> select index_name from user_Indexes where table_name = 'A1_TEST';

INDEX_NAME
------------------------------
SYS_C0051416

SQL> drop table a1_test;

Table dropped.

SQL> create table a1_test (id number);

Table created.

SQL> create index i1 on a1_test (id);

Index created.

SQL> alter table a1_test add constraint pk_test primary key (id);

Table altered.

SQL> select index_name from user_Indexes where table_name = 'A1_TEST';

INDEX_NAME
------------------------------
I1