Update one of 2 duplicates in an sql server database table

I have got a table that got a column with duplicate values. I would like to update one of a 2 duplicate values so for example row1 = tom and row2 = tom.. I want to add a 1 or an a to one of them and that will be for many other duplicates in the same column. Basically just add one number or letter to every 1 of the duplicates so there's no more duplicates.

I got this query that will update all the duplicates but not one of them. Can anyone help?

UPDATE Table1
   SET Column1 = 'a'
 WHERE exists
       (SELECT Column1 , COUNT(Column1 )
FROM Clients
GROUP BY Column1 
HAVING ( COUNT(Column1 ) > 1)
)