Does EE allow multiple members to use the same email address when registering? I'm able to register members using the same email address, which is something I don't want to happen. This Allow duplicate emails/no email address on Member Registration seems to suggest that EE does not allow it, though. Is this supposed to be a setting or something?
ExpressionEngine doesn't allow this by default. It's possible another add-on has built this into their product, but Safecracker Registration just combined member registrations with Safecracker to get the benefit of channel entries. So by design, Safecracker Registration is limited by what the native member module can do, at least as far the settings go.