The Twins 
Effects
Notes
- The following items/trinkets cannot be duplicated:
Synergies
- (Golden Trinket) Golden Trinket / (Mom's Box) Mom's Box: Has an additional 50% chance to trigger the effect again, resulting in another familiar doubling. This can be the same familiar that was doubled originally.
- (Sacrificial Altar) Sacrificial Altar: Both the duplicated familiar (if one is available) and Brother Bobby/Sister Maggy (if not available) can be sacrificed for an item reward. This does not remove the trinket, and fully retains its effect.
Interactions
- Movement-mimicking familiars: Duplicated familiars will follow the original on the same delay.
- In the case of (Shade) Shade, a copy of the familiar will spawn even if Shade was already absorbed by Isaac.
- Pickup-generating familiars: If they remain duplicated for enough consecutive rooms, they will give a pickup as normal. The following familiars have to be duplicated for the following number of consecutive room clears to give pickups:
- (1up!) 1up!: Despite the fact that it can be duplicated, Isaac will not keep the 1up if he dies while it's duplicated.
- (Bum Friend) Bum Friend / (Bumbo) Bumbo / (Dark Bum) Dark Bum / (Key Bum) Key Bum / (Lil Portal) Lil Portal: The pickup count of duplicated familiars is not remembered when they go away.
- (Ball of Bandages) Ball of Bandages / (Cube of Meat) Cube of Meat: Upgrades the familiar instead of duplicating it.
- (Best Bud) Best Bud / (Bird Cage) Bird Cage: If the trinket tries to duplicate it, it will instead immediately activate. Taking damage does not spawn another familiar.
- If this happens to Bird Cage as Isaac walks into a room with enemies, Bird Cage will also leap onto the enemies.
- (Blood Puppy) Blood Puppy / (Bumbo) Bumbo / (Guppy's Hair Ball) Guppy's Hair Ball: The duplicate will always be at its first stage.
- (Buddy in a Box) Buddy in a Box: The duplicated familiar will be randomly generated instead of copying the original.
- (Cambion Conception) Cambion Conception / (Immaculate Conception) Immaculate Conception: Familiars given by these items can be duplicated as normal.
- (GB Bug) GB Bug: The familiar can be duplicated even if it hasn’t respawned; by constantly moving to new rooms, GB Bug can constantly respawn, allowing pickups to be rerolled indefinitely.
- (Guillotine) Guillotine: The head can be duplicated, and will give an extra damage and tears up if it is. The extra head will be unable to fire tears.
- (Hallowed Ground) Hallowed Ground: Duplicate poops will also drop when Isaac takes damage. The bonuses from standing in the aura of multiple white poops does not stack.
- (Lost Soul) Lost Soul / (Star of Bethlehem) Star of Bethlehem: The trinket can try to make a duplicate of the familiar, but it will always fail. This reduces the odds of the trinket giving copies of other familiars or prevents it from giving Brother Bobby or Sister Maggy if Isaac has no other familiars.
- (Mystery Egg) Mystery Egg: The friendly the duplicated egg spawns depends on how many consecutive rooms cleared the duplicate persisted for.
- (Strawman) Strawman: Keeper cannot be duplicated, as he is considered a character, not a familiar.
- (Succubus) Succubus: The damage multiplier from standing in the aura of multiple Succubi multiplicatively stacks.
- (YO LISTEN!) YO LISTEN!: An extra +1 luck will be given for every duplicate.
???'s Soul
Apollyon's Best Friend’s unique attack locusts
Isaac's Head
Umbilical Cord
Beth's Essence, 
Beth's Faith, (Book of Virtues) Book of Virtues, (Jar of Wisps) Jar of Wisps, (Lemegeton) Lemegeton, or (Vengeful Spirit) Vengeful Spirit