SQL Server 2017

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create table YourTable (
  [ROW ID] int primary key identity(1,1), 
  [REFNO] int not null,
  [Amount 1] int not null,
  [Amount 2] int not null,
  [COUNT] int
);
insert into YourTable ([REFNO], [Amount 1], [Amount 2]) values
 (100000, 100, 200),(100000, 250, 250),(100000, 300, 300)
,(100000, 400, 400),(100000, 400, 100),(100000, 500, 100)

,(100010, 200, 100),(100010, 100, 300),(100010, 400, 400)

,(100021, 200, 100),(100021, 200, 200)

,(100032, 300, 300),(100032, 200, 100)

,(100033, 200, 100),(100033, 200, 100)
;
15 rows affected

WITH CTE AS
(
    SELECT *, CASE WHEN [Amount 1] = [Amount 2] THEN rn ELSE MAX(rn) OVER (PARTITION BY [REFNO] ORDER BY [ROW ID] ASC ROWS UNBOUNDED PRECEDING) END as rnk
	FROM (
	  SELECT [ROW ID], [REFNO], [Amount 1], [Amount 2], [COUNT],
	   IIF([Amount 1] = [Amount 2],1,0) as same,
	   CASE WHEN [Amount 1] = [Amount 2] THEN ROW_NUMBER() OVER (PARTITION BY [REFNO], IIF([Amount 1] = [Amount 2],0,1) ORDER BY [ROW ID] ASC) ELSE 0 END AS rn
	   FROM YourTable
    ) q
)
UPDATE CTE
SET [COUNT] = rnk;
15 rows affected

select * from YourTable order by [ROW ID];


ROW ID
REFNO
Amount 1
Amount 2
COUNT


1
100000
100
200
0

2
100000
250
250
1

3
100000
300
300
2

4
100000
400
400
3

5
100000
400
100
3

6
100000
500
100
3

7
100010
200
100
0

8
100010
100
300
0

9
100010
400
400
1

10
100021
200
100
0

11
100021
200
200
1

12
100032
300
300
1

13
100032
200
100
1

14
100033
200
100
0

15
100033
200
100
0

ROW ID	REFNO	Amount 1	Amount 2	COUNT
1	100000	100	200	0
2	100000	250	250	1
3	100000	300	300	2
4	100000	400	400	3
5	100000	400	100	3
6	100000	500	100	3
7	100010	200	100	0
8	100010	100	300	0
9	100010	400	400	1
10	100021	200	100	0
11	100021	200	200	1
12	100032	300	300	1
13	100032	200	100	1
14	100033	200	100	0
15	100033	200	100	0