create table aaa (id int unique, d text);

CREATE OR REPLACE FUNCTION tf_aaa_ignore_dup_row() RETURNS trigger LANGUAGE plpgsql AS $body$
BEGIN
  if exists(select * from aaa where id=NEW.id) then
    raise NOTICE 'row ignored: aaa.id=%', NEW.id;
    return NULL;
  end if;
  return NEW;
END;
$body$;

CREATE TRIGGER aaa_ignore_dup_row BEFORE INSERT
  ON aaa FOR EACH ROW
  EXECUTE PROCEDURE tf_aaa_ignore_dup_row();
  
insert into aaa select  1 , 'a';
insert into aaa select  2 , 'b';
insert into aaa select  2 , 'bb';
insert into aaa select  2 , 'bbb';
select * from aaa;