Looking back the the duels and the recent duel history buttons, I noticed a little annoying part for me: infinite revenges (counter attacks).
What to change about it? The names will be removed or the counter attack button is grayed on the name once the counter attack is done. At this point, I can attack lower leveled people for an easy 20 win streak and the upper leveled people can attack me for easy trophies to push themselves higher in the rankings.
Both of these loopholes really breaks the dueling system.
Also, I think there should be a much less award (or a friendly attack) for attacking people from the same Studio or in the friends list. That way, you will not need to rely on the above (attacking low for medals/win streaks) to progress up the ranks or finish the achievements. So all in all, the only place to receive full medal compensation will be inside the Duels page and the once only Counter Attack.