It´s against the rules. But those are the rules that many don´t comply with, so there are no controls with that.
Anyway, I don´t know if this affects the server by recharging.
Nor do I know how far it´s fair. It can happen that more than 1 account has the same IP address, because the computer is used by more than 1 person. It´s difficult to prove. In most cases it´s obvious that are multi-acc, but even the obvious cannot always be proven.