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OK. Mea culpa … perhaps! "This probability…." refers to the probability for sets with unique elements only. Probably other readers too think of "This probability…." to refer to the probability of the birthday paradox. Again, that's why I say that the birthday paradox is better understood via the mathematics of sets. More deeply, there are no formulae, but algorithms. Thus, the cases of absurdity for N > M can be avoided. The formula of arrangements (to calculate the number of sets with unique elements only) leads to an absurdity. You can't arrange 6 dice taken 7 at a time!