Unfortunately, SMF does not have a functional mod to disable the requirement for unique email addresses on account registration. I've checked, but it's integrated into how one logs in, and isn't easily removed without hampering the way the forum functions. See here:
http://www.simplemachines.org/community/index.php?topic=43142.0If you want to register multiple handles (for different characters or for a nation), simply use an alternate email address or use a bogus one (example:
email@internet.com). Just make sure you keep track of the passwords you use, if they are different, for each account so you're not stuck having to get me to reset it for you.
Keep in mind, you can post as an unregistered "guest", if you want to post as an NPC or something of that sort, although you'll lose the ability to edit or delete your posts after they're up.
An easy solution
would be to switch to a different forum software (like PHPBB), but I find SMF to be less cumbersome, takes up less unnecessary resources and isn't as glitchy as the latest and "greatest" incarnation of PHPBB, so I'm a bit hesitant to switch just for the ability to override the unique email requirement.