Projectile Duplication
Projectile Duplication is a perk that gives you a 50% chance to shoot a projectile twice, but causes you to take 25% more damage from projectiles.
The 50% chance is the same for all spells in a cast, so Triple Spell + 3 Spark Bolts can fire either 3 or 6 projectiles with one stack of the perk.
The effect is an extra modifier, only applied after spells of type Projectile, Static Projectile, or Material are normally cast ("Drawn").
All projectile modifiers apply to the duplicate projectiles, including cast and recharge delay, excluding mana drain.
The effect is applied after a spell finishes "executing". This means that typically, Trigger spells will not be copied, but rather, the last spell drawn when preparing their payload will be copied alongside the trigger projectile itself. This can lead to dangerous situations with spells such as Magical Explosion; e.g., Spark Bolt With Trigger + Magical Explosion with a single stack of the perk will either work normally or it will fire a spark bolt containing two magical explosions as a payload plus a magical explosion directly from the wand. This type of setup can also be used to duplicate Projectile Modifiers or other types of spells, but with additional complications due to Draw from these copied modifiers. This setup cannot be used to duplicate spells with zero charges, as these spells will be put in the Discard rather than the Hand, and the perk copies the last spell in the Hand.
Stacking
Stacking this perk increases the number of projectiles that are created when you successfully shoot additional projectiles.