assuming I understand what you're asking correctly. afaik it goes
gem cast vs prevent. if gem gets prevented it never happened.
if gem doesn't get prevented you then get gem vs 0.x% to multi. so if 1 gem gets through 1-4 gems can be coming at you but if you prevent the first one no others will follow.
Let me try to rephrase, not at all what I was trying to say. If the toon using the prevent gems has double cast gear, does the double cast affect prevents. Can the prevents trigger twice as a result of the double cast gear?
Double casting only happens when you, the caster, use a gem. Triggers won't activate it.
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Computing the probability that at least one of the following events will occur:
P(a or b ... or z) = 1 - P(!a and !b ... and !z) Probability