Additional context for IHT

I know this isn’t part of syllabus, or at least I think it isn’t, but when discussing the lifetime transfers of the nil rate band from deceased to the widow/widower, what would happen if if the widow remarried/found a new spouse or civil partner and they also unfortunately died. Could this transfer of the nil rate band continuously accrue over time to this single person depending on how many of their partners die, or would the transfer be lost upon remarriage/civil partnership to a new person?

Very morbid question I know but just wondering lol.

Thanks.