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Hmm, I seem to recall there is a general rule against receiving a plenary indulgence more than once a day.
But my main point was that I would think the indulgence is applied to a person's soul when he completes the indulgenced prayer or work, no? So this would mean that when you finish the prayer you get eight plenary indulgences applied to your soul at the same time. That's the problem here, I think.
Yeah, that's the general rule, but All Souls Day was one example of an exception. Pope could do whatever he wanted, i.e. allow 50 per day, but it does seem contrary to traditional practice to grant more than 1 per day.
I don't think that it was necessarily 8 times simultaneously. I read it as 8 times per day, where you had to say the prayers 8 times. But it could be simultaneous. Recall also that a Plenary isn't always plenary as applied to a given individual, since if one doesn't have freedom of attachment to sin, it would become partial. In addition, the "extra" ones could be applied to souls in Purgatory.
I'm more taken aback by the 14 million years "partial" ... though the 8 per day would also be rather unusual.