Will this do?

CREATE TABLE #WO(WO int, RD varchar(10))

INSERT INTO #WO VALUES (122 ,'asas')

INSERT INTO #WO VALUES (123 ,'der')

INSERT INTO #WO VALUES (124 ,'wer3er')

INSERT INTO #WO VALUES (124 ,'rtre')

INSERT INTO #WO VALUES (124 ,'hehehe')

SELECT *, CASE WHEN Row_no > 1 THEN

CONVERT(varchar, WO)+ CHAR(63 + Row_no)

ELSE CONVERT(varchar,WO) END FROM

(Select * ,

Row_Number() OVER (Partition BY WO ORDER BY WO) as Row_No

FROM #WO) #t1