Unless specified otherwise, spell effects do not stack.
When multiple spells affect the same aspect of a character the effects of the spells do not stack, and instead the spell with the highest Potency takes effect.
For example, a character benefitting from a spell that grants her +1 Strength casts a spell that grants her +3 Strength. The original spell with +1 Strength is suppressed for the Duration of the new spell. Both spell’s Durations continue to lapse, though only the highest Potency spell is active.
If the stronger spell lapses, the weaker spell reasserts itself until its remaining duration wears off.