Seeing Double

Seeing Double is challenge #40. The goal is to defeat Mom's Heart. This challenge is unlocked by defeating Mother.

Difficulty
Otherwise a very standard run, the challenge comes from every single enemy and boss placed naturally in rooms (as opposed to being spawned later) being doubled. Due to the high enemy density damage may prove difficult to avoid in some rooms. It is recommended to familiarize yourself with the gameplay of Jacob and Esau prior to attempting this challenge.

Strategy
Since libraries are able to spawn in this challenge, these can be a valuable resource when trying to beat the challenge. can help with protecting one of the brothers as well as instantly killing both Mom and Mom's Heart, will create helpful wisps in combination with other books (e.g. ), and  will help with sustaining health on the character with lower health.

is an extremely powerful item for this challenge that can be found in Shops, as it can prevent certain enemies from spawning for the rest of the run, so erasing enemies that are more common can help tremendously.

Mapping effects such as or  can prove valuable in this challenge, both to optimize charges on active items and to try and avoid dangerous rooms such as Mini-Boss Rooms and large rooms.

Teleportation cards (e.g. ), items (such as ), or Telepills can be used to quickly get out a room that appears challenging upon entering. The can also be used to return to the previous room and revert the health loss.

/ will help with clearing more dangerous rooms, though careful usage is required as some enemies can spawn multiple enemies upon death, potentially overwhelming the player. The card as well as others in the "Ace of" family can instantly clear rooms with large amounts of monsters. can be an effective boss killer, greatly damaging or eliminating 1 or both bosses. , if thrown correctly, can instantly kill both bosses in a single throw or used to kill other minor enemies as well.

can be incredibly useful to reduce monsters' health as soon as you enter the room. can launch a chain reaction, killing all the other enemies in the room, proving especially useful in this challenge because of the usually high density of monsters. Other area of effects tears like, , or  will benefit from. can be given to Esau to render him immortal while you take control of Jacob exclusively; be careful however, as some enemies will continue to target Esau, which will cause some enemies' patterns to become unpredictable.

Enemies and bosses in Challenge Rooms are not doubled, as they are not spawned when Isaac initially enters the room, so it's worth considering to clear them for extra pickups and items.

The Silver Dollar can be worth to hold onto as it will provide shops in Chapter 4.

Picking up flight as only one of the two characters can prove dangerous as this greatly increases the risk of separating Jacob from Esau.