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If you tag a bunch of mobs with this all at once then drop one quick it stacks the spread with the first one. If you then reapply growing swarm to a mob with one or the other or both dots stacked it overwrites. I have tested this.
I do not know exactly but def more then winters revenge. It will only spread once, the dot it spreads does not spread and the target that spreads it is not reinfected. Barring you tagging multiple targets at the first etc.
I think with a stam build this ability would go well with proc on dmg effects.