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.
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?