Can the nightmare effect of the Dream spell stack?

Question

Let's say a character casts the spell Dream, and chooses to make the messenger terrifying to cause a nightmare. Then, they deliver their ten words and wake up, and immediately cast Dream again on the same target to give them another nightmare during the same long rest. What happens? Does the target have to make a new Wisdom save for each casting? If they fail both, do they take 6d6 psychic damage upon waking?

Answer 1

Spells Don't Stack With Themselves

Effects of spells don't stack with themselves while the durations overlap:

The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect--such as the highest bonus--from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.

Since the duration (how long the spells effects last) is 8 hours, the effects of only one dream spell can effect the target at a time.

Second Casting

If they succeeded on the save, you can choose the end the spell early and try again.

The messenger can emerge from the trance at any time, ending the effect of the spell early.

Thus a second casting in that instance wouldn't have overlapping duration. But if the failed the save and you tried to double down on the damage, ending the first spell would end all the effects of the spell including the damage when they wake. A second nightmare won't deal extra damage, but could allow you to deliver 10 more words.

Answer 2

RAW, it sounds possible but I don't think it's RAI (Rule As Intended).

Spells don't stack. But technically, they won't:

The effects of the same spell cast multiple times don't combine [...] the most recent effect applies if the castings are equally potent and their durations overlap.

Here is a loophole: the two Dream spells wouldn't have their durations overlapping. The description of the spell states:

[...] the messenger appears in the target's dreams and can converse with the target as long as it remains asleep, through the duration of the spell

The messenger can emerge from the trance at any time, ending the effect of the spell early.

(about the nightmare part) [...] the messenger can deliver a message of no more than ten words and then the target must make a Wisdom saving throw

The messenger can deliver their 10 words and emerge, ending the spell. If the caster re-cast it, they wouldn't have 2 Dream spells active at the same time.

Since they are 2 different spells, both allow a saving throw.

Point of clarification

On a failed save, echoes of the phantasmal monstrosity spawn a nightmare that lasts the duration of the target's sleep and prevents the target from gaining any benefit from that rest. In addition, when the target wakes up, it takes 3d6 psychic damage.

A point of detail, but the "nightmare" have 2 dissociated effects : the "no rest" and the damages. The "no rest" might be considered as an effect of the spell that last the duration of the target's sleep but again, the spells won't have their duration overlapping.

It seems possible by RAW, but I doubt it's RAI.

A new point of view

After seeing J.A.Streich's comment, I though of a new way to explain how I look at the spell : Imagine the spell have a duration of "Instantaneous" but, when you cast the spell and the target doesn't sleep, the spell is in "stand by".

The spell never state that it need to be active for the full duration of the target sleep, which seems logic : if you casted the spell and the target don't sleep yet after 2 hours, the spell won't be up when the target wake up 6 hours later.