Say a creature walks through a door and triggers two spell glyphs from two glyph of warding spells that are programmed to cast a fireball at the creature that walks through the door. Both glyphs are triggered by the same action/trigger.
My intuition and off-the-cuff ruling would have been that the creature rolls for both fireballs and takes damage and effects as if they had been hit by two separate spells.
However, the PHB says in Chapter 10 under "Combining Magical Effects" (here in the basic rules):
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. (PHB p. 205)
Does this imply that the creature would only effectively get hit by one fireball? Does this section even apply here to spells of instantaneous duration?
In short, does the creature take damage from one or two fireballs when hit by two simultaneously?
A more extreme example of this scenario (and the inspiration for this question) can be found in: Can I create a superbomb with Glyph of Warding and a book?
They take damage from both fireballs
The key for me is the word "effects" from your quote:
I would argue that the damage dealt from the fireball is not an "effect" in the context of what your quote is talking about. This spell simply deals damage and has no additional "effects".
Furthermore (as pointed out by @PJRZ), the section you quote from begins with this sentence:
This suggests that spells that simply have an instantaneous effect do not come under this rule, this they effectively have no durations. This also confirms that the context I mention above is for ongoing effects of a spell, where considering overlapping bonuses or additional effects makes more sense.
Instantaneous isn't a real duration, it's just a category of duration for the purposes of that property of a spell
Instantaneous means it has no duration; as for the duration of a spell being listed as instantaneous, I understand duration in this context to simply mean the name of a category, a property of the spell, but it isn't actually a "duration" unless a number is given (or "Until dispelled"); i.e. until a period of time passes to make "duration" meaningful.
From PHB, pg. 203:
Further, Xanathar's Guide to Everything (pg. 77) has this to say about Simultaneous Effects (thanks to @V2Blast for suggesting this):
This also suggests that both fireballs should be resolved separately, making separate saving throws and damage rolls as though they did not happen simultaneously, but rather one after the other as though they happened a fraction of time apart.
Finally, the Essentials Kit includes some of these rules, but with a slightly different wording (thanks @Medix2). In the Essentials Kit Rulebook, on p. 3, under the section "Effects with the Same Name Don't Stack":
Durations is mentioned again, as in the PHB quote I included earlier in this answer (from p. 205), but in this case, what counts as a duration is made explicit; "a duration is a time span of 1 round or more". This also points to Instantaneous not being a real duration, further suggesting that both fireballs would deal damage.