The buff/debuff doesn't stack--it simply overwrites. However, a 10-second gap in the ATK+ rotation is something to the tune of 8000 points of damage lost in a crowded hamlet. The ATK bonus is like quintuple damage, so the stark difference in not having it active for even a short period vs. having it active 100% of the time can lead one to believe it's super-effective when overlapped (well, it is super-effective when overlapped, but not because the power is multiplied).
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