Update one of 2 duplicates in an sql server database table

Question

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

Answer 1

I think this simple update is what you're looking for;

UPDATE Table1 SET Column1=Column1+CAST(id AS VARCHAR)
WHERE id NOT IN (
  SELECT MIN(id) 
  FROM Table1
  GROUP BY Column1
);

Input:

(1,'A'),
(2,'B'),
(3,'A'),
(4,'C'),
(5,'C'),
(6,'A');

Output:

(1,'A'),
(2,'B'),
(3,'A3'),
(4,'C'),
(5,'C5'),
(6,'A6');

An SQLfiddle to test with.