SQL - consulta duplicados

DECLARE @MYTABLA1 TABLE (LETRA CHAR(2))

DECLARE @MYTABLA2 TABLE (LETRA CHAR(2))

insert into @MYTABLA1 values('a'),('a'),('b'),('c'),('d'),('d')

insert into @MYTABLA2 values('d'),('d'),('f'),('a'),('a'),('c')

SELECT LETRA FROM  (

select  LETRA,rn = row_number()

over(PARTITION BY LETRA  order by LETRA)

from    @MYTABLA1

Union

select  LETRA,rn = row_number()

over(PARTITION BY LETRA  order by LETRA)

from    @MYTABLA2

) T1

WHERE rn > 1

insert into @MYTABLA1 values('a'),('a'),('b'),('c'),('d'),('d')

insert into @MYTABLA2 values('d'),('d'),('f'),('a'),('a'),('c')