Need some sort of command like
.duel <playername> which could be additive (i.e. 4 v 4 duels, etc.
.duel player1
.duel player2
would allow you to fight these two players and no one else. Those two players would have to ".duel" you as well.
Anyone who came and shot you without dueling privileges would be killshot.
To avoid abuse (two players on same squad using this, then running around impervious to everyone else's bullets), you would not be allowed to shoot down any other players, either. You would also be kill shot for this.