Yeah, it's pretty much normal in Private Servers to kill a player through duels.. It happens when a last DoT tick occurs for a split second when the duel ends which kills the who lost.
I.e A warrior have Rend DoT on another duelling player, the other player loses the duel and for a split second right before the duel ended, the Rend DoT tick occurs which damages the player bypassing the faction neutral. It's normal.