Is there a mod that prevents a user from logging in with 2 different accounts from the same IP address in the same day?
Might be tricky.
If you block an IP address, you'll be blocking a whole range of them. Not just one, specific, machine.
Also what happens if you get 2 people from the same household as members?
An even better example the problems this would cause would be those on a company/school internet connection which can have several hundred people all on the same IP address - only one person could login per day!