The code doesn't actually write the same bytes in page 0 multiple times, it writes the boot vector and usb pin change vector while leaving the rest of the page reset to 0xff. Then it writes the rest later on, without touching the boot vector.
That's true, though if there was some reason to write 0's to the same byte in multiple phases, it could do that as well. I've done that with other applications, though I can't see a need to with this one.