In minecraft there is one option on some servers that u can put /pvp on and /pvp off, and it works only if both parts have it on. Then it is for sure consensual and can be duelled everywhere.
There are friends that like to duel for fun, or ppl that like to join games and find some other duellers. I think about the option in diablo2 where u can name a game “Duel” and ppl that likes it can join.
Or else I dont like the duel option in d2, especially in hardcore, it destroyed the hc mode for me, i only played in private passwordedv games and thats not always the fun.
If d4 will have a hardcore mode, then free duelling is not possible in that mode, and as many tells above - it really destroys an online game when u are into one quest and someone disrupt it that way. Not fun to leave a game when u have worked hard on some quest.
My tip is the minecraft version to toggle /pvp on and off - if they will implement pvp.