Can anyone explain the maths behind dual wielding to me?

Sample 1)
Mainhand = Weapon "One"
Off-Hand = No Weapon
Result = 100% Average Damage

Sample 2)
Mainhand = Weapon "One"
Off-Hand = Copy of Weapon "One" (exactly the same weapon, rarity "common")
Result = less than 100% Average Damage

Once again, can anyone explain this? All other stats (for example attack speed) keep the same. I don't get it.