pg_bulk_insert_hack.sql

create table table_data (
  code integer primary key not null,
  data_value varchar(30) not null
);

create table stage_data (
  id serial primary key,
  code integer not null,
  data_value varchar(30) not null
);

insert into stage_data (code, data_value)
values  (1, 'value 1'),
        (2, 'value 2'),
        (3, 'value 3'),
        (3, 'value 3 xx');

with ranked as (
  select
    code,
    data_value,
    rank() over (partition by code order by id desc) id_rank
  from stage_data
)
insert into table_data (code, data_value)
select code, data_value
from ranked
where id_rank = 1
on conflict (code)
do
		update
	  set data_value = excluded.data_value;

select *
from table_data;