Below is a link to a thread of a member here who recently trawled through all of the failed SN threads, along with a quote on re-dosing.
https://sanctioned-suicide.net/thre...-on-the-forum-as-a-cause-of-sn-failure.47986/
"I've only seen reports of a couple of members who were able to manage taking a second dose after vomiting. Everyone else seems to have succeeded regardless. And there is no reliable evidence at all that vomiting causes failure. So I don't see this as something to stress about."
It seems to me from the case studies we have that actually getting round to re-dosing is very unlikely. Having personally read through a number of the success threads as well I haven't seen one where anyone has re-dosed (but I haven't read them all, so there may be some examples I have missed). Obviously we can't know for sure if anyone who was successfully re-dosed without mentioning, as they aren't around to tell us - but if they have it hasn't been made clear from any of the threads I have seen.
I don't think there's any harm in preparing a second dose. But I think the evidence we have points to there being a slim chance of you actually needing one/having the chance to take it. And as mentioned in the thread I've linked, vomiting seems extremely unlikely to lead to failure as well - so I'm not sure taking a second dose would be productive.
I don't think anyone can reliably answer your question, because we don't have enough examples of people re-dosing in order to determine what best practice is. So sorry if I haven't answered your question - I've just tried to provide the most relevant information I can.