When using a pill, card, or rune, Echo Chamber activates one of every pill type, card type, and rune type that has been previously used since obtaining Echo Chamber.
Previously used cards, pills, and runes are effectively used again, meaning items that modify the effects of cards, pills, or runes will also modify the effects that Echo Chamber activates.
As a result, previously taken golden pills and blank runes will activate a random effect each time Echo Chamber activates them.
Regardless of how many times a single type of card, pill, or rune has been used previously, it will only activate the effect once.
Using items that convert pill effects into their good/bad counterparts such as PHD or False PHD can allow some pill effects to be activated twice by taking two different pills that activate the same effect.
If Isaac uses cards that convert/destroy pickups (such as Ace of Spades, Black Rune, IX - The Hermit?, etc.), any other cards Echo Chamber activates that generate pickups have a high chance to have their pickups converted.
It is advised to avoid using cards that teleport Isaac, as he will be teleported to a specific room every time he uses a consumable. This effectively negates any current-room effects from consumables, such as XV - The Devil or Rune of Algiz, as Isaac is teleported after they activate.
Similarly, it is also advised to avoid using unidentified pills, as Isaac runs the risk of activating either a stat down or Telepills every time he uses a consumable.
When obtained on Tainted Lazarus, Echo Chamber will only duplicate cards or pills previously used on the Lazarus state with Echo Chamber. As a result, teleport cards or Telepills can be freely used on the Lazarus state without Echo Chamber.
If an item is somehow removed and reobtained during the run (such as when obtained from Lemegeton or Modeling Clay), all of the consumable types used during the time Isaac had Echo Chamber will be remembered.
A card, rune, or pill must be consumed for the effect of Echo Chamber to trigger. That means using Blank Card, Placebo, or Clear Rune will not trigger Echo Chamber.
? Card + Ancient Recall / Vurp Pill + R Key: The ? Card effect can be used in conjunction with R Key without consuming it, allowing the player to in theory (with the help of Clicker) obtain completion marks for multiple routes and characters in a single run.
False PHD: Taking benign pills (e.g. puberty) will give black hearts or damage ups for each of the pills that are activated. Caution should be exercised when using this synergy, since all previously taken pills will be converted to their bad counterparts, and one stat down can ruin the synergy.
YAZ7 ZEP9 First secret room (left of room before boss)
1V1X PNQ6 Eden starts with this item
This is possibly a reference to the card Eye of the Storm from Magic: the Gathering, as the item resembles the art of the card and both result in previous effects being reused when a new one is activated.
This is also possibly a reference to the card Echo Form from 'Slay the Spire', which will duplicate the card effect of the first card played per turn.