Uppdatera en av 2 dubbletter i en sql-serverdatabastabell

Prova detta med CTE och PARTITION BY

;WITH cte AS
(
  SELECT
      ROW_NUMBER() OVER(PARTITION BY Column1  ORDER BY Column1 ) AS rno,
      Column1 
  FROM Clients
)

UPDATE cte SET Column1 =Column1 +' 1 '
WHERE rno=2