Just a quick, simple suggestion.
Wouldn't it be good to make it possible to challenge someone to a ranked duel with "f" and to unranked with, let's say "g"?
Why would we need this? Actually, many people come to duel server just for training. Guys like Atze may find it really frustrating, as he couldn't fight his really low lvl "friend" for fun. This could end up in a lose of many-so hard earned points.
What do you think?