Change Duplicate Data In Server Column

I want to change duplicate data in my sql server column.

the data= 'Barack Obama' , what I want
'Barack Obama 23453'

I have the following query:

UPDATE klant SET naamvoornaam=naamvoornaam + CAST(klantnummer AS VARCHAR)
WHERE naamvoornaam NOT IN (
SELECT(naamvoornaam)
FROM klant
GROUP BY naamvoornaam
HAVING ( COUNT(naamvoornaam) = 1 ))

Message from messages:
String or binary data would be truncated.

I think the max length exceeds the length of the destination column. I've tried:

Max(Len(naamvoornaam + CAST(klantnummer AS VARCHAR)))

message from messages:
An aggregate may not appear in the set list of an UPDATE statement.