Hi Graham, I agree with you that its a pain. However, its something that imo is very difficult to accurately identify in general, and even more difficult to do globally from the service POV. meaning... is the user actually asking for two things? or are they just saying one thing that is confused between two.I have always historically relied on disambiguation to have them choose the one thing they mean, and then they naturally ask for the second thing they wanted (if there were truly two) when they finish the first, and there really haven't been any major negative impacts from this method. I get its not ideal, but there's a balance between simplicity of the dialog design vs. end user experience.
Here's another good blog I wrote about identifying the optimal threshold, it should also explain how to set the threshold for a given dialog node: