Why subflow "booking" is called twice

[hawk]

I have run the example project (booking-mvc) inside spring-webflow distribution. I found when main flow enter sub-flow "booking" (when clicking "Book Hotel" button), it calls it twice. Does anyone know why?
I know it because I found creatBooking() is called twice.

Extracted from booking.xml :

<on-start>
    <evaluate expression="bookingService.createBooking(hotelId, currentUser.name)" result="flowScope.booking" />
</on-start>

[hawk]

I'm sorry that I post in wrong area.