Add clarification on single attribute rolls

dragonhack.adoc

@@ -308,6 +308,9 @@ The most dice you can roll in a challenge roll is 5.
 To create a dice pool for a challenge roll, you use attributes or other numbers, called a [term]#stat#, and roll an amount of dice equal to their rating.
 For example, if you roll with the [term]#heart# and [term]#wits# attributes and their value is 2 and 1 respectively, this is written as "Roll [term]#+heart+wits#" and you would roll 3D.
 
+When rolling with attributes, it's possible to be asked for a single attribute twice.
+This means you would roll twice the amount of dice for that attribute.
+
 [sidebar]
 If you ever need to roll zero or negative dice, roll w[2D].
 You cannot roll a critical hit when you have zero dice.