I casted Duel on Antimage when he had Aghanims and his Spell Sheild was not on cooldown. When I casted Duel his Spell Shield triggered and went on cooldown. The duel still started and I won. Something interesting about the situation is that I picked the 40+ duel victory damage in my level 25 Trait, and when I won I only got 18 damage. Is this a bug? Is it supposed to be this way?

Edit. Just realized he reflects the spell. Casting the Duel on me!