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

While it's never been shown in the series, I imagine you 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 you already sold it once, and it's no longer in your possession to do so.

-ahem- well, technecialy you 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. You only have one soul so you can only bring one other soul to hell

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