Private Sub cmdOK_Click()
Dim var As Variant
Dim intRow As Integer
var = Application.Match(CDbl(txtValue.Text), Columns(1), 0)
If Not IsError(var) Then
MsgBox "Wert ist bereits vorhanden!"
Else
intRow = Cells(Rows.Count, 1).End(xlUp).Row + 1
Cells(intRow, 1) = txtValue.Text
End If
End Sub