Can casting the Aid spell stack on the same targets?

Question

When you cast Aid at second level, it buffs three creatures' max HP and current HP by 5 for 8 hours.

Can this spell stack, either by casting it multiple times or by having several spell casters cast it on the same creature?

Answer 1

No.

The effects of the same spell cast multiple times don't combine.... For example, if two clerics cast bless on the same target, that character gains the spell's benefit only once. (PHB p.205, "Combining Magical Effects")

There are interesting cases, also covered in that PHB section, that arise when we consider durations and potency. But you don't mention those in your question, so I don't address them here.

Answer 2

No

Aid says:

Your spell bolsters your allies with toughness and resolve. Choose up to three creatures within range. Each target's hit point maximum and current hit points increase by 5 for the duration.

The increase in hit point and hit point maximum is for the duration. Aid has a duration of 8 hours, it is not an instant effect. This is different from a spell such as cure wounds: this is an instantaneous effect, so once the hit points of cure wounds are created, there is no duration to overlap, those hit points are non-magical and you get to keep them for good. There even is a an entry in the Sage Advice Compendium on this:

Can you use dispel magic on the creations of a spell like animate dead or affect those creations with antimagic field? Whenever you wonder whether a spell’s effects can be dispelled or suspended, you need to answer one question: is the spell’s duration instantaneous? If the answer is yes, there is nothing to dispel or suspend. Here’s why: the effects of an instantaneous spell are brought into being by magic, but the effects aren’t sustained by magic (see PH, 203). The magic flares for a split second and then vanishes. [...]
Another example: cure wounds instantaneously restores hit points to a creature. Because the spell’s duration is instantaneous, the restoration can’t be later dispelled. And you don’t suddenly lose hit points if you step into an antimagic field!

Because aid has a non-instantaneous duration, it's entire effect falls under the rule for Combining Magical Effects (p. 202, PH) apply:

The effects of different spells add together while the durations of those spells 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.
For example, if two clerics cast bless on the same target, that character gains the spell’s benefit only once; he or she doesn’t get to roll two bonus dice.

Both the granting of the extra hit points, and the granting of the increased maximum are suppressed if there are overlapping durations of aid and one of them has a more powerful effect.

Now aid says that "current hit points increase by 5 for the duration", but clearly, these are not indestrucible, everlasting hit points that nothing can take away during the duration. You are not immortal with always having 5 hit points for the entire duration, no matter what. Like any hit points, these hit points can be removed by taking damage. That however does not change the fact that aid granted you 5 extra current hit points for the duration.

When aid ends, your current hit points also go away again. That is, when you first cast aid, and then cast a more powerful aid at a higher level, you lose both 5 hp from your current total and lower the hp maximum by 5 as the original casting gets suppressed, and at the same time you get the new current hp and new increased hit point maximum.

(See also If I have a lower-level Aid cast on me, am reduced to less than 5 HP, and then have a higher-level Aid cast on me, what happens? for the specific situation where you have less then 5 hp as this happens.)

What is the "most potent" effect if both have the same power?

By the rules, a spells power is expressed by its spell level (PH, p. 201):

Every spell has a level from 0 to 9. A spell’s level is a general indicator of how powerful it is

If you cast a spell like false life that creates 1d4+4 temporary hit points at first level is cast two times, and one grants 5 temp hp, the other 8 temp hp, which of the two is the "most potent effect"? A natural reading is that the instance with the higher rolled outcome is a more potent effect, even though they have the same spell level. If the outcome is the same however, like with aid that provides a fixed 5 extra hit points, neither of them is the most potent one, so which one prevails?

There is another, more general rule in the DMG (p. 252) that cover simultaneous effects called Combining Game Effects

Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them-the most potent one-apply while the durations of the effects overlap. For example, if a target is ignited by a fire elemental's Fire Form trait, the ongoing fire damage doesn't increase if the burning target is subjected to that trait again. Game features include spells, class features, feats, racial traits, monster abilities, and magic items. See the related rule in the "Combining Magical Effects" section of chapter 10 in the Player's Handbook.

And there is yet another version in Xanathar's Guide to Everything (p.4)

Different game effects can affect a target at the same time. For example, two different benefits can give you a bonus to your Armor Class. But when two or more effects have the same proper name, only one of them (the most powerful one if their benefits aren't identical) applies while the durations of the effects overlap. For example, if bless is cast on you when you're still under the effect of an earlier bless, you gain the benefit of only one casting. Similarly, if you're in the radius of more than one Aura of Protection, you benefit only from the one that grants the highest bonus.

Neither of these says if the later or earlier effect applies during the time where both overlap. That strongly suggests that it should not matter which one prevails, you always only have the same, unchanged benefit.

If there was some tricky way to receive 5 extra current hp from each casting of aid and have them stick around when aid is gone, it would matter if you get to keep the first, or the second casting. But as both the current, and maximum hit points go away when the casting that is not active goes away, it does not.

Answer 3

It depends on how you think aid works

Aid is a mess; it can't work as written because current hp is not a base value to which ongoing continuous affects can be applied. It is clear that the increase in maximum hp is an ongoing effect, and if two aid spells were cast on the same target, the same-name effects rule would apply while their durations overlapped. What is not clear is how the effects on current hp would be resolved.

For maximum hp it is clear they do not stack

Sometimes a spell will produce multiple, independently existing effects, and in the case of aid there are two separate and distinct ones:

Each target's hit point maximum and current hit points increase by 5 for the duration.

One effect of the spell increases the hp maximum of the target, and an independent effect increases the current hp of the target (actually, if the casting includes all three of the targets permitted, that is six separate spell effects all produced by the same spell!).

The spell duration, as written, is 8 hours. If the effect of increasing maximum hp was still running from the first spell at the time the second spell was cast, then these are indeed simultaneous effects from spells of the same name, so in this case the more potent of the two would be in effect while the other was suppressed for the time their duration overlapped (if the spells were cast at equal level, the more recent one would be the more potent). Only one increase in maximum hp would be permitted - they would not stack.

For current hp, it depends on how you run the spell

For the increase in current hp, you might treat this as an ongoing effect that lasts for the duration of the spell and which continually modifies the current hp total (on this question, answers from doppelgreener, the Hobgoblin, and Kirt take this approach). If this is how you run the spell, then these increases in current hp are also overlapping effects of the same name, and if they were cast at the same level then the more recent one would suppress the older one. In this case, the second aid cast would appear to have no net effect other than increasing the effective duration.

Continuous effect with a hp 'loan'

The first two answers linked subscribe to a 'loan' theory, in which the aid spell increases your current hp when cast, but then actively decreases them at the end of its duration when the hp are 'taken back' by the spell. Despite this process not actually being in the spell description, it is necessary to understand the change in current hp as an ongoing effect. Interestingly, in this case, having a second aid spell running when the first one ended would interfere with the 'take back' process, since the first spell's effect would be suppressed. What you would see 'on the ground' is that the target's current hp would increase upon the casting of the first spell (but not the second), and then the target's current hp would decrease with the ending of the second spell (but not the first).

Now consider what would happen when two aid spells were cast in succession, but the first one was upcast and the second one not. A 3rd level aid is cast on the target, increasing its current hp by 10. Sometime later a 2nd level aid spell is cast on the same target. Now the target does not gain 5hp because even though this new casting is more recent, the first casting is 'more potent', so the lower-level spell is suppressed. An hour after the first casting it completes its duration, and the target drops 10hp as the spell somehow reclaims these, but immediately the target gains 5hp as the effect from the second cast is no longer suppressed, for a net drop of 5hp. Later, at the end of the duration of the second spell, the final 5hp are 'repaid' to the spell.

Continuous effect with "first out"

The third answer linked above does not require the hp granted by aid to be in the form of a loan. Rather, they are granted by the casting of the spell, but are always the first hp to be lost to any damage. At the end of the spell they are lost, but only if they have not already been depleted by damage. In this case, overlapping castings of the same level would suppress the older cast. This might result in a change in current hp: if the hp granted by the first spell had already been depleted, the casting of the second spell would appear to 'restore' them.

If hp gain is not a continuous effect

A second answer from Kirt on the same question linked above suggests running aid more in the manner of heroes' feast, where the spell itself has a duration, the effect of the increase in maximum hp has the same duration, but the increase in current hp is treated as a one-off instantaneous effect.

In this case, since these two effects of the same spell have different durations, they will interact differently with the identical effects of another aid spell. Or, as Thomas Markov once aptly put it, "...the DMG rule concerns the duration[s] of the effects, not just the listed spell duration."

If you ran aid spells this way, the first aid spell would (instantaneously) increase the current hp of the target while also increasing their maximum hp for the duration. The second aid spell would then increase the current hp of the target again. Because the first increase in hp had already happened and ended, the second increase does not apply the 'simultaneous effects' rule. This particular effect is not operating simultaneously for both spells. Nor would we say that these two effects 'stacked' in the strict sense of both applying at the same time - rather it was first one and then the other. (However, if you mean 'stack' in the loose sense of gaining the benefit of both spells during the time that their durations overlapped, then yes they would)