Make sure spirit guardians don't stack.

[Kyso4ek]

A creature should be only hit once per round by effects such as spirit guardian etc.

Same with the other aoe spells that might be cast on top of each other.

[Heu, Iterum Id Feci]

This has been a Topic of Discussion in the D&D Community for quite some time, and there are good arguments on both sides.
Yes, RAW you are most likely right, and Spirit Guardians shouldn't stack.
However, there have been points made, that this rule is intended for de-/buffs, not for dmg effects. I personally as a gm would probably handle this on a case by case basis and would try to favour whatever brings the players the most options for their play.
ie the damage from both spirit guardians would apply, however the speed debuff only applies once. The Players invested the spell slots, so they'd get the effects. However in such a campaign NPCs would use the same tactics against the players then (while watching that I don't overdo it as a gm).
I've also seen this ruling from other GMs or in arguments.
However, I also always was in the lucky position of my players not abusing things like this, but using it because it made sense for their characters. Would I have players that abused mechanics like that, I would absolutely stop that. Not via ruling, but by an encounter showing up that abuses it the same way, they did, just in a more thorough manner. Usually that helps them understand the point. ;)

[Kyso4ek]

Originally posted by (っ◔◡◔)っ ♥ Rusty ♥:

This has been a Topic of Discussion in the D&D Community for quite some time, and there are good arguments on both sides.
Yes, RAW you are most likely right, and Spirit Guardians shouldn't stack.
However, there have been points made, that this rule is intended for de-/buffs, not for dmg effects. I personally as a gm would probably handle this on a case by case basis and would try to favour whatever brings the players the most options for their play.
ie the damage from both spirit guardians would apply, however the speed debuff only applies once. The Players invested the spell slots, so they'd get the effects. However in such a campaign NPCs would use the same tactics against the players then (while watching that I don't overdo it as a gm).
I've also seen this ruling from other GMs or in arguments.
However, I also always was in the lucky position of my players not abusing things like this, but using it because it made sense for their characters. Would I have players that abused mechanics like that, I would absolutely stop that. Not via ruling, but by an encounter showing up that abuses it the same way, they did, just in a more thorough manner. Usually that helps them understand the point. ;)

Investing into spell slots multiple guardians allows people to cast the spell wherever required. It's good enough.

Just thought of 4 clerics with spirits destroying everything that approaches. Obvious rule leak

[Cartesian Duelist]

There is no reason within the rules that damage from multiple Spirit Guardians would not stack. If two wizards shot a target with lightning bolts would you cancel the damage from one?

[Heu, Iterum Id Feci]

Originally posted by Cartesian Duelist:

There is no reason within the rules that damage from multiple Spirit Guardians would not stack. If two wizards shot a target with lightning bolts would you cancel the damage from one?

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

. (DMG p. 252)

Edit:
If your copy of the DMG doesn't have that ruling see the Official Errata on the start of page 2:
https://media.wizards.com/2018/dnd/downloads/DMG-Errata.pdf

[Cartesian Duelist]

Originally posted by (っ◔◡◔)っ ♥ Rusty ♥:

Originally posted by Cartesian Duelist:

There is no reason within the rules that damage from multiple Spirit Guardians would not stack. If two wizards shot a target with lightning bolts would you cancel the damage from one?

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

. (DMG p. 252)

Since damage is an instantaneous effect, there is no overlap. Spirit Guardians doesn't cause ongoing damage, it's a new save for a new damage roll every round. Lightning bolt doesn't cause ongoing damage, sneak attack doesn't cause ongoing damage, extra attack doesn't cause ongoing damage, breath weapon doesn't cause ongoing damage.

[Heu, Iterum Id Feci]

Originally posted by Cartesian Duelist:

Originally posted by (っ◔◡◔)っ ♥ Rusty ♥:

. (DMG p. 252)

Since damage is an instantaneous effect, there is no overlap. Spirit Guardians doesn't cause ongoing damage, it's a new save for a new damage roll every round. Lightning bolt doesn't cause ongoing damage, sneak attack doesn't cause ongoing damage, extra attack doesn't cause ongoing damage, breath weapon doesn't cause ongoing damage.

However, the condition is, that it enters the Spirit Guardians area, and that is an effect, which also slows enemies down. As this can't stack the damage won't get triggered, and as such RAW you can't get damage from 2 Spirit Guardians.

Edit:

Spirit Guardians

3rd-level conjuration

Casting Time: 1 action

Range: Self (15-foot radius)

Components: V, S, M (a holy symbol)

Duration: Concentration, up to 10 minutes

You call forth spirits to protect you. They flit around you to a distance of 15 feet for the duration. If you are good or neutral, their spectral form appears angelic or fey (your choice). If you are evil, they appear fiendish.

When you cast this spell, you can designate any number of creatures you can see to be unaffected by it. An affected creature’s speed is halved in the area, and when the creature enters the area for the first time on a turn or starts its turn there, it must make a Wisdom saving throw. On a failed save, the creature takes 3d8 radiant damage (if you are good or neutral) or 3d8 necrotic damage (if you are evil). On a successful save, the creature takes half as much damage.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d8 for each slot level above 3rd.

There it's pretty clear, that it is an area effect. However any area can only ever have 1 active effect of the same name, ergo no 2 Spirit Guardians triggering at the same time, if you go by RAW.

[Kyso4ek]

Originally posted by Cartesian Duelist:

There is no reason within the rules that damage from multiple Spirit Guardians would not stack. If two wizards shot a target with lightning bolts would you cancel the damage from one?

This should only work once for persistent effects.

Two greases are overkill.
Two drinking clouds should give only one save
Two cloud kills
Two webs
Two evards
Two spirit guardians etc

Otherwise the game provokes infinite stacking of clouds in top of clouds it's broken. Five webs on top of each WILL beat legendary saves in no time

Remember killing firkraraag with a million traps stacked on top of one another? How stupid was that

Just thinking what happens when there are too many effects of the same type and this produces times more area damage or sav d than the spell was designed to deal.

To the point there are insta kills.

[Kyso4ek]

When you place a web. There s no benefit to place another web because the web is not going to be improved.
This game grid hex still is classified ad covered in web

[Cartesian Duelist]

Originally posted by (っ◔◡◔)っ ♥ Rusty ♥:

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

. (DMG p. 252)

See this quote that you referenced as being in the 5E DMG on page 252? You should probably check that because it doesn't exist. Page 252 of the DMG contains optional rules for facing and diagonals and some rules for running chases. Why don't you find out where this actually comes from. I'm holding the book in my hands right now and the passage you reference isn't there. But just in case, here are a couple links for you to see for yourself.

https://online.anyflip.com/ofsj/axvy/mobile/index.html#p=252
https://archive.org/details/dungeon-masters-guide/Dungeon%20Master%27s%20Guide/page/n251/mode/2up

[Cartesian Duelist]

Originally posted by Kyso4ek:

Originally posted by Cartesian Duelist:

There is no reason within the rules that damage from multiple Spirit Guardians would not stack. If two wizards shot a target with lightning bolts would you cancel the damage from one?

This should only work once for persistent effects.

Damage isn't a persistent effect.

Originally posted by Kyso4ek:

When you place a web. There s no benefit to place another web because the web is not going to be improved.
This game grid hex still is classified ad covered in web

Correct, because Web causes the Restrained condition and conditions only stack duration and not effect. Web doesn't cause damage.

[Heu, Iterum Id Feci]

Originally posted by Cartesian Duelist:

Damage isn't a persistent effect.

No, the Spirit Guardians Damage isn't a persistent effect. However, the Spirit Guardians Area that triggers the damage roll is a persistent effect. As those can't overlap, only the stronger one is active at any given area, and thus the damage can only be triggered once.

Originally posted by Cartesian Duelist:

Correct, because Web causes the Restrained condition and conditions only stack duration and not effect. Web doesn't cause damage.

And Spirit guardians slow the target and trigger the damage roll Neither the slow nor the trigger stack. Only the most potent one can be active at the same time.

[Heu, Iterum Id Feci]

Originally posted by Cartesian Duelist:

Originally posted by (っ◔◡◔)っ ♥ Rusty ♥:

. (DMG p. 252)

See this quote that you referenced as being in the 5E DMG on page 252? You should probably check that because it doesn't exist. Page 252 of the DMG contains optional rules for facing and diagonals and some rules for running chases. Why don't you find out where this actually comes from. I'm holding the book in my hands right now and the passage you reference isn't there. But just in case, here are a couple links for you to see for yourself.

https://online.anyflip.com/ofsj/axvy/mobile/index.html#p=252
https://archive.org/details/dungeon-masters-guide/Dungeon%20Master%27s%20Guide/page/n251/mode/2up

Interesting...Both my DMG and D&D Beyond do have that passage just before the Chases Headline.
However, if you go to the PHB in your second link On page 186/187 (Chapter 10, I can't decipher the original Page number, sry) You'll find pretty much the same ruling there under the headline "Combining Magical Effects".
I assume your pdf is from one of the earlier editions of the dmg, and later editions have this passage added.

Edit:
Link to the passage in the DMG on DDB:
https://www.dndbeyond.com/sources/dmg/running-the-game#CombiningGameEffects
Link to the passage in the PHB on DDB:
https://www.dndbeyond.com/sources/phb/spellcasting#CombiningMagicalEffects
They obviously only work, if you own those nooks on DDB, which is why I originally didn't want to link to DDB.

Edit 2:
You can also find it in the DMG Errata on the top of the second page:
https://media.wizards.com/2018/dnd/downloads/DMG-Errata.pdf