if object_id('[tb]') is not null drop table [tb]
go
create table [tb]([NAME] varchar(1),[QUANTITY] int)
insert [tb]
select 'a',1 union all
select 'a',1 union all
select 'b',2 union all
select 'b',2 union all
select 'a',1 union all
select 'a',1 union all
select 'b',2 union all
select 'b',2

;with t1 as
(select rn=row_number() over(order by getdate()),* from [tb]),
t2 as
(select rn-(select count(1) from t1 where name=t.name and rn<=t.rn) gid,* from t1 t)

SELECT NAME,SUM(QUANTITY) AS QUANTITY
FROM T2
GROUP BY NAME,GID
ORDER BY MIN(RN)

/**
NAME QUANTITY
---- -----------
a    2
b    4
a    2
b    4

(4 行受影响)
**/

------解决方案--------------------

--tb为原来的表，tmp为加了序号的临时表
select id = identity(int,1,1) into tmp from tb

select name , sum(QUANTITY) QUANTITY from tmp group by name , (id-1) / 2