When you are importing user with Drupal, you probably do it with user import module, but you'll probably have a list of uses without any email or with duplicate emails, in this module its hardcoded that one email is used per person. However if you install shared emails module and do some hacking on current DEV version (2.x) you will be able to do that. In modules "supported" folder there is user.inc and go to line 235, there is a code that says
// Handle duplicate emails.
elseif (!$duplicates_allowed && _user_import_existing_uid($email)) {
user_import_errors(t('duplicate email'));
}
and just delete it, this is probably a code that should in some future permit (from user interface) to have this option but as its not active currently this will help you do this.