;with cte as
(
SELECT House_acc
FROM  yourtable
GROUP BY House_acc
HAVING (COUNT(DISTINCT repcode) > 1)
)


SELECT A.House_acc , 
       A.AccountId , 
       A.repcode
  FROM
       cte INNER JOIN yourtable AS A ON cte.House_acc = A.House_acc;

SELECT A.House_acc , 
       A.AccountId , 
       A.repcode
  FROM
    (
        SELECT House_acc
        FROM  yourtable
        GROUP BY House_acc
        HAVING (COUNT(DISTINCT repcode) > 1)
    ) dt
     INNER JOIN yourtable AS A
        ON dt.House_acc = A.House_acc;

DECLARE @H TABLE
    (House_Acc INT, Accountid INT, repcode VARCHAR(10))

INSERT INTO @H
SELECT 123, 1, 'J978A'
UNION ALL SELECT 123, 2, 'J978A'
UNION ALL SELECT 123, 3, 'J978A'
UNION ALL SELECT 123, 4, 'EG567'
UNION ALL SELECT 456, 21, 'BR5TG'
UNION ALL SELECT 456, 22, 'BR5TG'
UNION ALL SELECT 678, 66, 'ZHR06'
UNION ALL SELECT 678, 45, 'ZHR06'
UNION ALL SELECT 678, 34, 'NH678'

SELECT a.House_Acc, b.AccountID, b.repcode
FROM (
    SELECT House_Acc, AccountID, repcode
        ,m=MAX(repcode) OVER (PARTITION BY House_Acc)
    FROM @H) a
INNER JOIN @H b
    ON a.House_acc = b.House_acc
WHERE m <> a.repcode