It properly gives the damage when the loser has Reincarnation (WK ulti) or Aegis. It does not give the damage if the loser has both

Repro:
1. Pick Legion Commander, lvl up ulti and get a view rapiers
2. Create an enemy Wraith King, give him an Aegis and level up his ulti
3. Duel the Wraith King and be victorious


Result: No Duel victory damage is granted for the victory.

Expected: Duel damage is granted for the victory.