How does spellcasting interact with Improved Duplicity?

Question

Trickery Clerics get this Channel Divinity option: Invoke Duplicity.

... As an action, you create a perfect illusion of yourself that lasts for 1 minute, or until you lose your concentration (as if you were concentrating on a spell). The illusion appears in an unoccupied space that you can see within 30 feet of you. ... it must remain within 120 feet of you. For the duration, you can cast spells as though you were in the illusion's space, but you must use your own senses. Additionally, when both you and your illusion are within 5 feet of a creature that can see the illusion, you have advantage on attack rolls against that creature, given how distracting the illusion is to the target.

This upgrades at 17th level: Improved Duplicity.

... you can create up to four duplicates of yourself, instead of one, when you use Invoke Duplicity. As a bonus action on your turn, you can move any number of them up to 30 feet, to a maximum range of 120 feet.

How does spellcasting interact with Improved Duplicity? In other words, could the spell originate from multiple (perhaps all four) duplicities at the same time, or from only one?

Answer 1

Invoke Duplicity only changes the location from which you cast spells.

Note the ability description:

you can cast spells as though you were in the illusion's space.

The only thing this ability changes is the point of origin of the spell you are casting. Nothing else changes about any spell or the rules for Spellcasting. A spell that targets one creature still only targets one creature. A spell that makes one melee spell attack still makes one melee spell attack.

Again, for emphasis, the only thing that changes is the location you are casting from.

Allowing a cleric to cast a spell through all four duplicates at the same time is so very obviously broken.

This is easily observed by examining any of the clerics damage or healing spells.

Let us consider guiding bolt:

A flash of light streaks toward a creature of your choice within range. Make a ranged spell attack against the target. On a hit, the target takes 4d6 radiant damage, and the next attack roll made against this target before the end of your next turn has advantage, thanks to the mystical dim light glittering on the target until then.

Cast at 1st level, if permitted to be cast through each duplicate, has the potential to do up to 16d6 damage, with the 2nd, 3rd, and 4th attack rolls being made at advantage if made against a single target.

Alternatively, consider the modest cure wounds:

A creature you touch regains a number of hit points equal to 1d8 + your spellcasting ability modifier. This spell has no effect on undead or constructs.

We would turn this into 4d8+20 healing, which is almost as strong as a Supereme Potion of Healing for only a 1st level spell slot.

And of course, upcasting these spells only exacerbates the issue.

Therefore:

You may select one of your duplicates and cast a spell as if you were in that duplicate's space.