As of 0.77, many spells such as obscuring fog stack. I think the cumulative effect of multiple instances of buff or debuff spells is imbalancing, at least without the availability of a dispel magic.
However, if the stacking of identical buffs and debuffs is intended, please display the stacking on the unit's tactical unit card with either an icon for each instance of the spell cast on a unit or a little "x2" or "x3" on top of the spell icon.
