加入网聊后，MS Bot迎来了两次

Question

我有以下情况：通过网页中的网络聊天提供机器人。加载网页时，机器人会问候并立即显示用户可以选择的一些选项。这是在MessagesController中使用的代码：

    [ResponseType(typeof(void))]
    public virtual async Task<HttpResponseMessage> Post([FromBody] Activity activity)
    {
        if (activity.Type == ActivityTypes.Message || activity.Type == ActivityTypes.ConversationUpdate)
        {
            if (activity.Type == ActivityTypes.ConversationUpdate &&
               !activity.MembersAdded.Any(r => r.Name == "Bot"))
                return Request.CreateResponse(System.Net.HttpStatusCode.OK);
            await Conversation.SendAsync(activity, () =>
            new RootDialog());
        }
        else
        {
            HandleSystemMessage(activity);
        }
        var response = Request.CreateResponse(System.Net.HttpStatusCode.OK);
        return response;
    }

并且是RootDialog类的一部分：

[Serializable]
public class RootDialog : IDialog<object>
{        
public async Task StartAsync(IDialogContext context)
    {
        var message = context.MakeMessage();
        var attachment = GetHeroCard();
        message.Attachments.Add(attachment);
        await context.PostAsync(message);
        context.Wait(this.ShowFirstCategoryRefinement);
    }
    public virtual async Task ShowFirstCategoryRefinement(IDialogContext context, IAwaitable<IMessageActivity> activity)
    {
        var message = await activity;
        IEnumerable<string> options = this.mainCategories;

        PromptDialog.Choice(
            context: context,
            resume: ChoiceReceivedAsync,
            options: options,
            prompt: "Please select product category:",
            retry: "Selected category not available. Please try again.",
            promptStyle: PromptStyle.Auto
            );
    }
...
}

实际发生的是：用户打开页面，机器人问候并显示一些选项。当用户选择一个选项时，机器人再次问候并显示相同的选项。如果用户再次选择一个选项，则对话框会正常继续。

如何解决这个问题？感谢。