FOR SELECT ...
  FROM Table
  ORDER BY Field  /* обязательно чтобы выбрать одинаковые записи последовательно */ 
  INTO ...
  AS CURSOR name
BEGIN
.....
END

DELETE FROM TABLE WHERE CURRENT OF name

ALTER TABLE Table
  ADD IDfield integer;

UPDATE Table
  SET IDfield = GEN_ID(generator, 1 );

DELETE FROM Table
WHERE IDfield not in (    
    Select MIN(IDfield) as MinID
    FROM TABLE
    GROUP BY oldField
)

CREATE TABLE tmp (
ID integer,
cnt integer
);

INSERT INTO tmp (ID,cnt)
Select MIN(IDfield) as MinID, count(IDfield) as CNT
   FROM TABLE
   GROUP BY oldField
;

DELETE FROM TABLE
  WHERE (IDfield not in (SELECT ID FROM tmp WHERE CNT =  1 ))
      AND (IDfield not in (SELECT ID FROM tmp))

DELETE FROM Table
WHERE IDfield not in (    
    Select MAX(IDfield) as MaxID
    FROM TABLE
    GROUP BY oldField
)