I saw this somewhere but can't find it now

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Anyone know what the hack is to block people from registering if they are from the same IP as another user? (Probably them).

I figure this would be good to help prevent dupe users, and if in the rare case that somebody in the same house as another user wants to sign up I could always manually register their account for them through the admin cp??? Well does anyone know where this hack is? I could have sworn I saw it before here. Thanks.