Outlook ItemAdd事件为新的日历项触发两次

我正在开发一个Outlook插件，它可以监视当前用户的日历，并在收到特定类型的约会或会议时向该用户发送电子邮件。我们有一个第三方应用程序/服务，它正在向Outlook中的用户发送新的会议请求，但没有通知给登录到Outlook的用户。我的插件是一个变通办法，直到我们取代第三方应用程序，所以当这个会议请求被发送时，用户可以得到提醒。

我使用ItemAdd事件来监控何时添加约会/会议(即从第三方应用程序发送)。我看到的是，事件触发两次(即使我只声明了一次处理程序):一次当从不同用户接收到约会时，一次当当前用户接受或暂时接受约会时一次。

我需要它只在第一次收到约会时触发，而不是在它被接受时。我可以监控用户的收件箱，看看他们是否已经收到通知，但我认为如果他们在点击接受之前没有收到电子邮件(服务器延迟?)，这就不太好了。

这是我的代码。如有任何意见，不胜感激。

public partial class ThisAddIn
{
    Outlook.Items _Appointments = null;
    Outlook.Folder _MyAppointmentsFolder = null;
    private void ThisAddIn_Startup(object sender, System.EventArgs e)
    {
        // Initialization.
        _MyAppointmentsFolder = (Outlook.Folder)this.Application.Session.GetDefaultFolder(Outlook.OlDefaultFolders.olFolderCalendar);
        _Appointments = _MyAppointmentsFolder.Items;
        _Appointments.ItemAdd += new Outlook.ItemsEvents_ItemAddEventHandler(appointments_Add);
    }
    private void appointments_Add(object item)
    {
        // An appointment has been added. Read the title and send an email based on a condition.
        Outlook.AppointmentItem meetingItem = item as Outlook.AppointmentItem;
        if (meetingItem.Subject.Contains("Service Call"))
        {
            // Let's send ourselves an email.
            string emailTo = string.Format("{0}@concurrency.com", Environment.UserName);
            string subject = meetingItem.Subject;
            string body = meetingItem.Body;
            string startDate = meetingItem.Start.ToString();
            string endDate = meetingItem.End.ToString();
            SendEmailAlert(emailTo, subject, body, startDate, endDate);
        }
    }
    ....

...
string _PreviousMeetingId = string.Empty; // class-level variable
...
private void appointments_Add(object item)
    {
        // An appointment has been added. Read the title and send an email based on a condition.
        Outlook.AppointmentItem meetingItem = item as Outlook.AppointmentItem;
        if (meetingItem.Subject.Contains("Service Call") && _PreviousMeetingId != meetingItem.GlobalAppointmentID)
        {
            // Let's send ourselves an email.
            string emailTo = string.Format("{0}@concurrency.com", Environment.UserName);
            string subject = meetingItem.Subject;
            string body = meetingItem.Body;
            string startDate = meetingItem.Start.ToString();
            string endDate = meetingItem.End.ToString();
            SendEmailAlert(emailTo, subject, body, startDate, endDate);
            // Save the ID of the meeting so we can check it later (above).
            _PreviousMeetingId = meetingItem.GlobalAppointmentID;
        }
    }