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 Booming Blade, which has the effect of dealing a bit of damage, and then leaving a lingering effect that deals extra damage if the target moves voluntarily on their turn. Lets say we’re a sorcerer who cast Booming Blade on an enemy, and then for good measure, we cast it again with Quickened Spell Metamagic. Then our Eldritch Knight ally decides that he really wants to be sure, and strikes with Booming Blade himself, then Action Surges to do it again. Is our poor foe subject to 4 instances of Booming Blade if it so dares to move voluntarily?
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?