Invincible can show up twice on the action bar. I believe the problem is caused by the fact that it is unlocked by the level ability even though the ability already auto unlocks.

The ability that grants Invincible reads-

<GameModifier>
<ModType>Unit</ModType>
<Attribute>UnlockCombatAbility</Attribute>
<StrVal>Invincible</StrVal>
<Provides>Caster is immune to physical damage until the next action</Provides>
</GameModifier>

Invincible's ability should read as-

<GameModifier>
<ModType>Unit</ModType>
<Provides>Caster is immune to physical damage until the next action</Provides>
</GameModifier>