Multiple asynchronous calls to the webservice method in an application fail

Question

I am trying to create an application that connects to the Internet and consumes web servicesfor each of them. As far as I know, I would like to use async method, which I already use to get a collection of contacts. I realized that when I do the same for groups (which means getting async groups), I get errors in calls, but when using a regular call there ins't.So I did some research on the Internet and find that many people have the same problem.
Some of them are invited to use WCF(for which I do not know jack). I will be interested to know if there is another way to overcome this. if someone can point me to a reliable resource on the internet and help me get through this? thanks for reading and help here is my code:

 public partial class main : Window
{
    //...
    private static vmcSession session;
    private MyService service = new MyService();
    private contactInfo[] loadedcontact;
    //....

    public main()
    {
        InitializeComponent();
        //service.addContactCompleted +=new addContactCompletedEventHandler(addContactCompleted);
        service.getContactsCompleted += new getContactsCompletedEventHandler(getContactsCompleted);
        service.getGroupsCompleted += new getGroupsCompletedEventHandler(getGroupsCompleted);
        fillContents();
    }

    private void getGroupsCompleted(object sender, getGroupsCompletedEventArgs e)
    {
        try
        {
            groupListBox.ItemsSource = e.Result;
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.InnerException.Message);
        }
    }

    private void getContactsCompleted(object sender, getContactsCompletedEventArgs e)
    {
        try
        {
            loadedcontact = e.Result;
            contactListBox.ItemsSource = loadedcontact;
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
    }

    private void addContactCompleted(object sender, addContactCompletedEventArgs e)
    {
        throw new NotImplementedException();
    }

    public void fillContents()
    {
        displayUserInformation();
        loadContacts();
        service.getGroupsAsync(session.key, null);
        //groupListBox.ItemsSource = service.getGroups(session.key, null);
        cmbSenderIds.ItemsSource = service.getSenderIds(session.key, null);
        if (cmbSenderIds.Items.Count > 0)
        {
            cmbSenderIds.SelectedIndex = 0;
        }
        loadGrid();


    }


    public void loadContacts()
    {
        service.getContactsAsync(session.key, null);
    }



    public void displayUserInformation()
    {
        lblName.Content = session.user.firstName;
        lblEmail.Content = session.user.email;
        lblCreditValue.Content = Decimal.Ceiling(session.user.balance).ToString();

    }


    public void loadGrid()
    {
        try
        {
            hitoryGrid.ItemsSource = service.viewMessages(session.key, null).ToList();
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
    }     

}

Answer 1

. 2 , Unique ID.each , GUID it.thanks