Sure, but what if two humans share an email address, but both of them want to sign up for your game/service/whatever independently? This is a fairly common sit

Sure, but what if two humans share an email address, but both of them want to sign up for your game/service/whatever independently? This is a fairly common situation if you are (say) designing a game that might appeal to kids. Or old people. If you block that, you're going to lose users. Either because they just can't sign up at all, or because there's too much unnecessary friction in requiring someone to go out and sign up for an email address just to register for your service.