Comment 14 for bug 1126560

- Why lo is missing on /etc/init.d/networking restart.
  - Because we never bring down "lo" on networking restart.

 - Why eth0 was registered 2 times (if-pre-up.d) on rebooting.
 No idea and I can't reproduce this here.
 Here I see the pre-up scripts being called 3 times:
  - once for --all
  - once for eth0
  - once for lo

- The difference between sequential executing (on restarting with /etc/init.d/networking restart) and parallel executing (on rebooting the system).
 The only difference should be the absence of "lo" as state above. My best guess for why you're getting eth0 twice is that of a race condition between the event based bring up and the standard init based bringup.