CD key violation

[Zooko]

Ugh, in the 2 vs 2 vs 2 vs 2 vs 2 game that I'm hosting and playing in, I got this message for one of the two nations that I am playing: "CD Key violation. Your CD key has been used on multiple installations. Your orders have not been executed.".

Some questions:

1. Why did this happen this time and not any of the other times?

2. Why did it happen for one of the two nations that I'm playing (my own), and not for my brother's nation for whom I'm subbing in while he is away?

3. Can I just turn off networking during the "checking your cd key" moment to avoid this? Hm, actually it's probably being done on the server. Maybe I should configure networking on the server to disallow that check...