select * from test 

Ref_id    cust_id  Amount
-------   -------  ------
1           1        100
1           1        100
2           1         50

cust_id  Amount
-------  ------
1        250

select * from test 

Ref_id    cust_id  Amount
-------   -------  ------
1           1        100
1           1        100
2           1         50

Expected Result

cust_id  Amount
-------  ------
1        250

SQL> WITH DATA AS(
  2  SELECT 1 cust_id, 100 amount FROM dual UNION ALL
  3  SELECT 1,        100 FROM dual UNION ALL
  4  SELECT 1,         50 FROM dual)
  5  SELECT cust_id, sum(amount) total_amount
  6  FROM DATA
  7  GROUP BY cust_id
  8  /

   CUST_ID TOTAL_AMOUNT
---------- ------------
         1          250

SQL>