Private Sub Text1_KeyDown(KeyCode As Integer, Shift As Integer)
 
If InStr(1, Text1.Text, ",") > 0 Then
 
If Chr(KeyCode) = "," Then
MsgBox ("Zeichen bereits vorhanden")
KeyCode = 0
End If
 
End If
 
End Sub

Listen to Reggae and Chill!
Bob Marley 4-ever!