Stronger copied attacks
What changes
Lilith's Incubus starts without the ordinary damage reduction, so BFFS! raises its damage from one to two times her damage.
How to approach the setup
Keep Incubus aligned with an enemy while dodging. After taking BFFS!, judge the familiar's hits as an additional damage source rather than expecting Isaac's HUD damage to double.
A practical suggestion for this pair. Check the rest of your inventory and the room before relying on it.
The caution to remember
The multiplier concerns Incubus, not every attack Isaac makes. Character rules and additional weapon modifiers still matter; these two starting cases do not calculate a complete inventory.
What this teaches about a run
A familiar upgrade can improve a run without changing the player's displayed damage. Check who is actually producing the attack.