id     name
1       a
1       a
2       b
2       b
2       b
3       c
4       d

SQL> create table tbl
  2    as
  3      select 1 id,'a' name from dual union all
  4      select 1,'a' from dual union all
  5      select 2,'b' from dual union all
  6      select 2,'b' from dual union all
  7      select 2,'b' from dual union all
  8      select 3,'c' from dual union all
  9      select 4,'d' from dual
 10  /

Table created.

SQL> alter table tbl
  2    add constraint tbl_pk
  3      primary key(id)
  4  /
  add constraint tbl_pk
                 *
ERROR at line 2:
ORA-02437: cannot validate (SCOTT.TBL_PK) - primary key violated


SQL> create index tbl_pk
  2    on tbl(id)
  3  /

Index created.

SQL> alter table tbl
  2    add constraint tbl_pk
  3      primary key(id)
  4      using index tbl_pk
  5  /
  add constraint tbl_pk
                 *
ERROR at line 2:
ORA-02437: cannot validate (SCOTT.TBL_PK) - primary key violated


SQL> alter table tbl
  2    add constraint tbl_pk
  3      primary key(id)
  4      using index tbl_pk
  5      novalidate
  6  /

Table altered.

SQL> insert
  2    into tbl
  3    values(5,'e')
  4  /

1 row created.

SQL> /
insert
*
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.TBL_PK) violated


SQL>