Using COM port in two or more windows

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C#

COM

WPF

Hello.
I have application with several windows (actually its two almost same windows with some functions). One of that function is reading data from COM port (read weight from weighting machine). Problem is that, it works perfect on first window, but when im closing it and opening another window, then clicking same function (lets call it calculate weight), im getting this error message:
Access to the port 'COM1' is denied.

Code:

//clicking button 
private void calculateWeight_Click(object sender, RoutedEventArgs e)
{
    sp = new SerialPort("COM1", 9600, Parity.None, 8, StopBits.One);
    tekst = string.Empty;
    sp.Open();
    sp.WriteLine(((char)5).ToString());
    sp.WriteLine(((char)17).ToString());
    sp.DataReceived += new SerialDataReceivedEventHandler(sp_DataReceived);
}

private delegate void UpdateUi(string s);
// entering there if get anything from serial port
void sp_DataReceived(object sender, SerialDataReceivedEventArgs e)
{
    tekst += sp.ReadExisting();
    if (tekst.Contains('S') && tekst.Length > 14)
        Dispatcher.Invoke(DispatcherPriority.Send, new UpdateUi(czytajWage), tekst);
}

string tekst = string.Empty;
//read all data reveived from port and show messagebox with it
void czytajWage(string s)
{
    string w = "";
    for (int i = 5; i < 14; i++)
    {
        w += s[i];
    }
    MessageBox.Show(w);
    w = "";
    tekst = "";

    sp.DataReceived -= sp_DataReceived;

}

Anyone can help? ;)