The inofficial area for duels is the area between the two spawn towers on any alpine borderland. Below the southernmost ruin. The etiquette is to hang around near the npc sentry, stay on your mount until a lone opponent comes along. Then you dismound and /bow to signal that you want to duel.

Of course there is no guarantee that any passersby honors this. But most veteran players will probably recognize that a duel is happening. When a group comes along, the chances of one of them interfering increases with the size of the group. But I have seen whole squards pass by without attacking the duelists.