If a spell has an instantaneous duration, but an effect that lingers, can that effect be stacked?

Question

I've seen this popping up for quite a few specific spells in the past, so lets get a general clarification down.

Overlapping effects says:

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, or the most recent effect applies if the castings are equally potent and their durations overlap.

The part that seems to be tripping up most askers is the "while the durations overlap" text, since some spells have a duration of Instantaneous.

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One of the classic examples would be Awaken, which has the effect of granting the target a increase to intelligence and the ability to speak a language. Disregarding how we can target a creature a second time with the spell, does casting it again grant them even more intelligence, along with an additional language?

Or another example, let's say that our Cleric has cast Ceremony, and chooses Wedding (+2AC, 7 days), then casts it again on both of the newlyweds, and chooses Dedication (+1d4 to saves, 24 hours). Does our newly married and dedicated couple gain both benefits?

So to put it plainly, since instantaneous effects have a duration of, well, Instantaneous, should the spells listed duration be taken as the effects duration in regards to stacking, or should any additional duration listed in the spell, be it 1 round or 1 year, be what is primarily considered?

If it's the latter, are there any that specifically break this rule?

Answer 1

No. It is the duration of the effect that matters

The rules for Combining Game Effects state:

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.

This section from the Dungeon Master's Guide addresses the duration of all kinds of game effects. The word duration in this rule refers to the duration of the effect, and with respect to spells, cannot be assumed only to reference the "Duration" property of spells.

The confusion here stems from the quoted portion of the Player's Handbook:

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, or the most recent effect applies if the castings are equally potent and their durations overlap.

Spells have a particular property "Duration", and it isn't entirely clear if this particular section from the PHB is addressing only that, or if it is also addressing the effects of the spells as well. That said, it is not unclear at all that the DMG rule concerns the duration of the effects, not just the listed spell duration.