-----------------
|   А   |   B   |
-----------------
|   2   |   1   |
|   3   |   4   |
|   2   | null  |
|   3   |   4   |
|   2   |   6   |
|   4   |   4   |
-----------------

for cdup in (select А, max(rowid) as rd
  from table
    group by А
    having count(1) > 1) loop
  delete from table where rowid <> cdup.rd;
end loop;