Why does Cortana speak twice?

Question

Just got to my home pc, it's time to play with what I learned at Build!

The code below is my default response for my Cortana skill. For some reason she says "Sorry. I don't know what you mean." twice. She isn't speaking the alt text for the card image. So... why is she saying it twice and how can I convince her once is enough?

    [LuisIntent("None")]
    private async Task None(IDialogContext context, LuisResult result)
    {

        var reply = context.MakeMessage();
        reply.Speak = "Sorry. I don't know what you mean.";
        reply.InputHint = InputHints.IgnoringInput;
        var card = new HeroCard
        {
            Images =
                new List<CardImage>()
                {
                    new CardImage("http://i.imgur.com/PiowyjZ.jpg", "Sorry, I didn't catch what you meant.")
                }
        };
        reply.Attachments.Add(card.ToAttachment());


        // return our reply to the user
        await context.PostAsync(reply);

        context.Wait(MessageReceived);
    }

Edit: She isn't doing it to me on Android, and after trying on a second PC it isn't happening there either. Problem is with Cortana on this PC, I guess.