Debuffs generally fall into 5 types: A, B, C, eidolon/Burst effect, stacking (not many of these). Debuffs of different classes stack additively, while debuffs of the same class are supposed to apply the strongest one. Except we verified that this isn't the case; a weaker debuff will remain and be renewed if overwritten with a stronger one. The stronger debuff will be active for the turn that it was applied, but in the big scheme of things that doesn't really help.