If Not ItemExists(List1, daten(schleife, 0)) Then
  List1.AddItem daten(schleife, 0)       
End If

Public Function ItemExists(oControl As Object, ByVal sItem As String) As Boolean
  On Error Resume Next
  oControl.Text = sItem
  If Err.Number <> 0 Or oControl.ListIndex < 0 Then
    ItemExists = False
  Else
    ItemExists = True
  End If
  On Error GoTo 0
End Function