Probably a more efficient way, but this is the idea:

; WITH MyCTE (acc_no, c_name, cnt)

AS

( SELECT acc_no

, c_name

, COUNT(c_name)

FROM #testing a

GROUP BY acc_no

, c_name

)

SELECT

t.acc_no

, c.c_name

, number_sum = SUM( t.number)

, r_value_sum = SUM( t.R_Value)

FROM #TESTING t

INNER JOIN mycte c

ON t.acc_no = c.acc_no

WHERE c.cnt = (SELECT MAX(d.cnt)

FROM MyCTE d

WHERE d.acc_no = c.acc_no

)

GROUP BY t.acc_no

, c.c_name

You need to count and get the max count of the names, by account and then join with the sum.