By a duel of wills do you mean like a staring contest or something similar. Or say two contestants dueling under the Accords to mentally push a piece of enchanted Mordite at each other?
By RAW, I would say its a Discipline vs Discipline contest where you could use other skills for declarations or maneuvers, like Intimidation to place a "Cowed" aspect or Deceit for a "Not expecting this" aspect.
*Edit: And the brain kicked in, yea this is a bit of head-scratcher. I think how the Duel of Wills is initiated will dictate how it proceeds. A staring contest could be a single exchange of Discpline rolls, but something more complex, it could be well anything.*