Valid If - for existing row

I have a “valid if” condition whether a customer number exists. It works for a new entry but not when I edit the row. What can i do?

NOT(IN([KdNr] , kunden[KdNr]))

NOT(
  IN(
    [KdNr],
    SELECT(
      kunden[KdNr],
      ([_ROWNUMBER] <> [_THISROW].[_ROWNUMBER])
    )
  )
)
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Thanks a lot!

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