sql>create table test (pk int primary key, uniq int not null unique);
 
Table created.
 
sql>declare
  2    v_pk    test.pk%type;
  3    v_uniq  test.uniq%type;
  4  begin
  5    for i in 1..4 loop
  6      if i < 4 then
  7        v_uniq := i * 10;
  8      else
  9        v_uniq := 20;  -- force a unique violation
 10      end if;
 11      insert into test (pk, uniq) values (i, v_uniq);
 12    end loop;  
 13  exception
 14    when dup_val_on_index then
 15      select pk
 16        into v_pk
 17        from test
 18       where uniq = v_uniq;
 19      dbms_output.put_line('Attemped to enter: ' || v_uniq || '  Already exists for pk: ' || v_pk);
 20  end;
 21  /
 
Attemped to enter: 20  Already exists for pk: 2
 
PL/SQL procedure successfully completed.