SQL Server 2017

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table t (Rownumber int,OldIdassigned varchar(100),commoncode varchar(100));
insert into t values(1,'FLEX','Y2573F102'),
(2,'RCL','Y2573F102'),(3,'FLEX','Y2573F102'),
(4,'QGEN','N72482123'),(5,'QGEN','N72482123'),
(6,'QGEN','N72482123'),(7,'RACE','N72482123'),
(8,'CLB','N22717107'),(9,'CLB','N22717107'),
(10,'CLB','N22717107')

10 rows affected

delete t2
from
(select t1.*, 
row_number() over(partition by commoncode,OldIdassigned order by  Rownumber) rn from t t1
)t2 where rn<>1
;
5 rows affected

select * from t


Rownumber
OldIdassigned
commoncode


1
FLEX
Y2573F102

2
RCL
Y2573F102

4
QGEN
N72482123

7
RACE
N72482123

8
CLB
N22717107

Rownumber	OldIdassigned	commoncode
1	FLEX	Y2573F102
2	RCL	Y2573F102
4	QGEN	N72482123
7	RACE	N72482123
8	CLB	N22717107