If you're installing it for different users, in different houses, no trouble (as far I know).
If you're using in the same house, you can use the same for more than one computer (if all are non-commercial).
About Yahoo, you confirmed that it works.
About the different key, oh, could be the date of the old ones. You'll receive the same key if you register at the same time with the same email, as far I tested.