Query to get duplicate records then update it by putting a sequence number
create table data 
(
  first_name varchar2(20 char),
  last_name varchar2(20 char),
  sequence_number number(10, 0)
);

insert into data values ('John', 'Smith', null);
insert into data values ('John', 'Smith', null);
insert into data values ('Jane', 'Doe', null);
insert into data values ('Jane', 'Doe', null);
insert into data values ('Jane', 'Doe', null);
insert into data values ('Rock', 'Lee', null);
commit;

select * from data;

FIRST_NAME           LAST_NAME            SEQUENCE_NUMBER
-------------------- -------------------- ---------------
John                 Smith                               
John                 Smith                               
Jane                 Doe                                 
Jane                 Doe                                 
Jane                 Doe                                 
Rock                 Lee      


--   

select
  first_name, last_name, rowid,
  row_number() over (partition by first_name, last_name order by rowid) as sequence_number
from data
  where (first_name, last_name) in 
  (select first_name, last_name from 
    data group by first_name, last_name having count(*) &gt; 1
  )
;

FIRST_NAME           LAST_NAME            ROWID              SEQUENCE_NUMBER
-------------------- -------------------- ------------------ ---------------
Jane                 Doe                  AAAJ7hAAEAAAAFjAAC               1
Jane                 Doe                  AAAJ7hAAEAAAAFjAAD               2
Jane                 Doe                  AAAJ7hAAEAAAAFjAAE               3
John                 Smith                AAAJ7hAAEAAAAFjAAA               1
John                 Smith                AAAJ7hAAEAAAAFjAAB               2


--

merge into data using
(select
  first_name, last_name, rowid,
  row_number() over (partition by first_name, last_name order by rowid) as sequence_number
from data
  where (first_name, last_name) in 
  (select first_name, last_name from 
    data group by first_name, last_name having count(*) &gt; 1
  )
) data_with_sequences
on (data.rowid = data_with_sequences.rowid)
when matched then update set data.sequence_number = data_with_sequences.sequence_number;

5 rows merged.


--

commit;
select * from data;

FIRST_NAME           LAST_NAME            SEQUENCE_NUMBER
-------------------- -------------------- ---------------
John                 Smith                              1
John                 Smith                              2
Jane                 Doe                                1
Jane                 Doe                                2
Jane                 Doe                                3
Rock                 Lee