If u Pull The String Once Can u Do it Again?

While it's never been shown in the series, I imagine u probably can't. Ai has denied some requests, so she'd probably deny one of someone who has already used it once. I think it's like trying to sell your soul, only u already sold it once, and it's no longer in your possession to do so.

-ahem- well, technecialy u can't really go to hell twice(you know like"to dig a double grave") so I don't think so

I think it's one soul for another. u only have one soul so u can only bring one other soul to hell

Well, if u send someone to hell u need to pay of what u have done, u shall be sent to hell too, but that's after u die. u only have one soul therefore, u can only send a person once. Enma Ai would probably, no she will really deny your request.