Buffs that don't stack visibly/functionally (we tie the two together) can still stack in the background. Instead of "refreshing" the duration of an existing buff, a new buff is added with its own duration. The buffs overlap on their duration, so the unit will (usually) enjoy the benefits of that buff longer, but the actual impact of the buff (+50% Offense, +25% Critical Chance, etc) is constant - if any of that buff is present, you get the static effect. The same is true for debuffs that don't stack visibly/functionally.
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So an example would be the following:
T1: Apply Tenacity Up for 2 turns
T2: Apply Tenacity Up for 2 turns again somehow
T3: Tenacity Up from T1 expires. AA bonuses. Character still has Tenacity Up
T4: Tenacity Up from T2 expires. AA Bonuses. Character no longer has Tenacity Up
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TLDR; leia and akbar omis work well together