help on update Statement on SAME Rows

I have table TblLandRiverSegment

RiverSegment          CbSegId   

EL0_4591_0000       80             

EL0_4591_0000        81            

XYZ_5570_0017        91           

EL1_5570_0001        91

EL1_5570_0002       NULL
EL1_5570_0002        73

 I am trying to update with help of self join
UPDATE  T
    SET T.CbsegId =S.CbsegId
    FROM TblLandRiverSegment T
    JOIN  TblLandRiverSegment S
    ON S.RiverSegment = T.RiverSegment

for your ease

CREATE TABLE TestSegment
  ( RiverSegment  Varchar(100) ,    CbSegId INT )
 
INSERT INTO   TestSegment
VALUES('EL0_4591_0000', 80),
('EL0_4591_0000',81)



    UPDATE  T
    SET T.CbsegId =S.CbsegId
    FROM TestSegment t
    JOIN  TestSegment S
    ON S.RiverSegment = T.RiverSegment
     SELECT * FROM TestSegment

If you look at the first 2 rows the  riversegment are same but cbsegid are different, when i update this update wont work  technically as update will not know what record to update ..it runs but will not update technically....so in this situation i need to throw error Raise error more records to update on multiple cbsegid have same landsegment some thing like that ..

Please help me ..

Thanks