It would refresh the duration back up to 18 seconds, EXCEPT if the previous cast was more potent. For example, if you cast Aero with Cleric's Stance up, and then 5 seconds later cast Aero without Cleric's Stance, Aero will only inflict the 50 potency direct damage, and will NOT inflict the DoT.

That said, multiple uses of Aero from DIFFERENT conjurers will stack.