I have uploaded a version of DuelResponse which will hopefully work with the WoW Classic Beta.
The only problem is I am not in said Beta, so I cannot guarantee if anything will work; if you do use it and find any problems, please do not hesitate to open a ticket. (I'm trying to pay more attention to the ticket tracker, as I received one back in JANUARY but wasn't notified about it... unless I was and simply forgot.)
Whether you love or hate to duel, DuelResponse has tools to make life easier.
Hate dueling? DuelResponse will block any and all duel or Pet Battle requests sent your way, allowing you to play in peace.
If you love dueling, then the addon can still help you in a number of ways:
Lagging? Decline duels while your latency is above a set number.
Frame rate gone to heck? Decline duels while you shout at your computer to stop acting up (well, that's what I do).
Resurrection Sickness? No duels for you!
AFK or Busy? Again, block all incoming duels until you return or finish doing what you're doing.
DuelResponse also has optional plugins which extend its functionality:
Blacklist is like an ignore list for duels; add someone to this, and all challenges they send will be declined. Handy for the kinds of people who like to get all up in your face and spam you with requests.
The whisper plugin, as its name implies, sends customised whisper messages to people when a duel is declined.