The wording is confusing but the concept is simple enough.
Versatile weapons default to two-handed if no shield is equipped so you cannot duel with them. And for the record using a versatile weapon with a shield does not reduce its damage - it does its default damage, e.g. a longsword does 1d8 one-handed and 1d10 two-handed. So its damage is increased if you use it two-handed.
In a nutshell: if the weapon does not have the label 'versatile' and if the off-hand is empty then you can use duel.
Far too conditional for my liking. Can't see much use for it unless you are going for a specific build.
