Duels as in mini-battles?
No no. Duels in the sense of, one vs one at the realm level. So, the rest of the empire is honour bound to intervene...To keep anyone else from intervening. You fight from the beginning of spring to the end of fall, and whichever realm wins takes the higher of the two ranks up for grabs. So, if there are three realms, and the third realm challenges the second realm, the top realm is obligated to fight, if necessary, to keep anyone else from getting involved. At the end of the war, the third realm is determined to be the winner, and thus becomes the second realm.