Important to know since some buff/debuff overwrites each other..
I couldn't find a good list, so I just list buff/debuff that I know of:
Stackable:
*Vanguard Spirit (Cleric, target)/Greater Protective Aura (Cleric, squad) + Aura of the Golden Bell (Blademaster, squad) + Verdant Shell (Mystic, self) + Energy Leech Devil Chihyu (Mystic, self) => stackable physical defense buff
*Ironheart Blessing (Cleric, target) + Sunder (Barbarian, self) => stackable heal over time
(IH also stacks with itself up to 5 times)
*Strength of the Titans (Barbarian, squad) + Sparks (Blademaster/Barbarian/Archer/Assassin/Seeker, self) + Heaven's Wrath (Cleric, squad) => increased attack buff
*Vital Herb (Mystic, squad) + Healing Herb (Mystic, squad) + Falling Petals (Mystic, target) => stackable heal over time
*Regeneration Aura (Cleric, squad) + Invoke the Spirit (Barbarian, self) => stackable bless (decreased damage taken)
*All Damage Over Time with same icons (need testing between Veno's Noxious Gas + Mystic's Swirling Mist)
*Magic Shell (Cleric, target)/Aegis Spirit (Cleric, squad) + Energy Leech Storm Mistress (Mystic, self) => increased magic defense
*Spirit's Gift (Cleric, target)/Arcane Empowerment (Cleric, squad) + Energy Leech Storm Mistress (Mystic, self) => increased magic attack
*Saber Rattle/Bladed Fervor (Seeker, squad) + Black Voodoo (Psychic, self) + Jones Blessing/O'Malley's Blessing => increased attack level
*Krav Maga/Bladed Fervor (Seeker, squad) + White Voodoo (Psychic, self) + Tideform (Assassin/Psychic, self) + O'Malley's Blessing => increased defense level
*Ressurection (Mystic, target) + Revive (Cleric, target) => res on spot (priority based on exp then hp/mp recover after dying)
*Beast King's Inspiration (Barbarian, squad) + Tree of Protection (Genie) => increased maximum HP
*Stormrage Eagleon (Archer, target) => slow debuff (stacks with itself)
Non-stackable:
*Amplify Damage (Venomancer, target) + Soul Degeneration (Venomancer, target) + Crush Vigor (Venomancer, target) => curse debuff
*Bladed Fervor (Seeker, squad) + Saber Rattle (Seeker, squad) + Krav Maga (Seeker, squad) => increased attack/defense level
*Ironwood Scarab (Venomancer, target) + Befuddling Creeper (Mystic, target) => reduced physical defense debuff
*Heaven's Flame (Blademaster, target) + Parasitic Nova (Venomancer, target) + Blood Vow (Archer, target) => chaos debuff
*Heaven's Wrath (Cleric, squad) + Demonic Eruption => non-stackable increased attack speed/channeling
*Regeneration Aura (Cleric, squad) + Celestial Eruption => non-stackable bless (decreased damage taken)
*Life Powder (apoth) + Celestial Guardian Seal (Cleric, target)/Exalted Renewal (Cleric, squad) => increased HP recovery
*Heaven's Wrath (Cleric, squad) DOES NOT stack with Cyclone Heel (Blademaster, self) + SAGE Slipstream Strike (Assassin, self) + Relentless Courage (Genie) + Wind Shield (Genie) => increased attack speed
*Frostblade (Wizard, target) + Condensed Thorn (Assassin, self) => add water damage to physical attack
*Alter Marrow Physical (Blademaster, self) + Alter Marrow Magical (Blademaster, self) => alternate debuff between increase/decrease physical and magical defenses
*Black Voodoo (Psychic, self) + White Voodoo (Psychic, self) => alternate buff between increase/decrease attack and defense levels
*Soul of Stunning (Psychic, self) + Soul of Retaliation (Psychic, self) => alternate buff between stun OR absorb/reflect damage, shares same cooldown
*Empowered Vigor (Psychic, target) + Diminished Vigor (Psychic, target) => alternate buff/debuff that increase/reduce hp recovery and charm cooldown, shares same cooldown
Need more testing on:
*Ironheart Blessing (Cleric, target) + Bubble of Life (Psychic, squad) => heal over time
Some people say it doesn't stack, but I never know if it's true or not.. I need to know this so I don't accidentally heal the tanker while tanker getting spammed by IH.. Both buffs have same icons..
*Soul of Vengeance (Psychic, target) + Bramble Guard/Bramble Hood (Venomancer, target) => reflect physical damage
Different icons.. Most likely stack.. But never know unless we do some testing..
*Rapid Growth (Mystic, self) + Demonic Eruption => increased channeling/magic attack
*Rapid Growth (Mystic, self) + Heaven's Wrath (Cleric, squad) => increased channeling/magic attack
*Tide Spirit (Psychic, self) + Demonic Eruption => increased channeling/magic attack
*Tide Spirit (Psychic, self) + Heaven's Wrath (Cleric, squad) => increased channeling/magic attack
*Flesh Ream (Barbarian, target) + Red Tide (Psychic, target) => bleed debuff
Bleed has same icon but Psychic's bleed is soulforce based while Barb's is not..