If a creature is effected by the same spell multiple times, the PHB tells you the effects of those multiple castings don't combine, and that's fair. But let's say the same spell is cast one something multiple times and it has an effect that the target can make a saving throw to break every round.
For the sake of argument, let's say I used Conjure Woodland Beings to conjure 8 Pixies. Let's say I am fighting a Big Bad and I really want it to be confused. It's got a high saving throw, so I have all 8 Pixies cast their Confusion spell on it in hopes that at least one will land. Then let's say that the Big Bad actually ends up failing 4 of it's 8 saves. So now the Big Bad is subject to 4 Confusion spells.
"At the end of each of its turns, an affected target can make a Wisdom saving throw. If it succeeds, this effect ends for that target." Is our Big Bad separately rolling 4 saving throws for the 4 Confusion spells on it? Does it have to steadily chip away at all of them before the Confusion finally ends? Or since multiple castings of the same spell can only effect something once, is one successful save all it needs?
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.
So by Rules As Written only the most recent Confusion would still be in effect, so the Big Bad only rolls one save at the end of its turn. If for some reason one of the pixies had a higher Spell Save DC than the others and the Big Bad failed the initial save against it, that one's casting of Confusion would be the most potent, and stay in effect.
If a creature is effected by the same spell multiple times, the PHB tells you the effects of those multiple castings don't combine, and that's fair. But let's say the same spell is cast one something multiple times and it has an effect that the target can make a saving throw to break every round.
For the sake of argument, let's say I used Conjure Woodland Beings to conjure 8 Pixies. Let's say I am fighting a Big Bad and I really want it to be confused. It's got a high saving throw, so I have all 8 Pixies cast their Confusion spell on it in hopes that at least one will land. Then let's say that the Big Bad actually ends up failing 4 of it's 8 saves. So now the Big Bad is subject to 4 Confusion spells.
"At the end of each of its turns, an affected target can make a Wisdom saving throw. If it succeeds, this effect ends for that target." Is our Big Bad separately rolling 4 saving throws for the 4 Confusion spells on it? Does it have to steadily chip away at all of them before the Confusion finally ends? Or since multiple castings of the same spell can only effect something once, is one successful save all it needs?
(And yes this isn't the best use of these 8 Pixies, I just needed a concrete example.)
So by Rules As Written only the most recent Confusion would still be in effect, so the Big Bad only rolls one save at the end of its turn. If for some reason one of the pixies had a higher Spell Save DC than the others and the Big Bad failed the initial save against it, that one's casting of Confusion would be the most potent, and stay in effect.
Helpful rewriter of Japanese->English translation and delver into software codebases (she/e/they)