DDuel is a minimalistic addon that will prevent players that aren't in your guild or part of your friends list from sending duel requests. The addon has no impact on your duel requests towards other players and can be toggled on or off with a simple slash command at any time, meaning it won't bother you if you're thinking of going outside a city for some 1 on 1 fun.
More in-depth information about this addon can be found in the download's Readme file.
Automatically declines duel requests from people not in your friends or guild list.
Notifies you of every declined duel via system messages in your primary chat frame.
Notifies the player that tried to duel you via whisper (these whispers will not appear for you).
Configurable options via slash command /dduel.
Ruby Sanctum & Patch 3.3.5:
It seems to be working fine with the latest patch, if you find an error or conflict with another addon, please let me know via comments or PM.
Players using DDuel who get refused by another player also using DDuel, will not receive the whisper notice (rather situational).