Concentration Spells

The following spells are concentration spells. If it's not on this list, it is no longer a concentration spell
Players' Handbook
Arcane Eye
Beast Sense
Clairvoyance
Conjure Elemental
Conjure Fey
Detect spells (Detect Magic, Good/Evil, Poison, Thoughts, etc)
Greater Invisibility
Hex
Hunter's Mark
Locate Creature
Locate Object
Phantasmal Killer
Scrying
Witchbolt

Xanathar's Guide
Beast Bond
Elemental Bane
Immolation
Infernal Calling
Summon Greater Demon
Summon Lesser Demons

Tasha's Cauldron
All Summon spells

Notes

Just because those spells don't require concentration, doesn't mean you can stack multiple versions. If a paladin casts Searing Smite on turn 1, then Elemental Weapon on turn 3, Elemental Weapon replaces the Smite. Same thing if my paladin casts a Smite spell, then a druid casts Flame Blade on the paladin's weapon, the second spell replaces the first. A cleric cannot cast Protection from Poison, then Protection from Energy, but two different clerics could cast them. Multiple spells of the same kind (such as Bless) do not stack their bonuses, but different bonuses (Bless and Bardic Inspiration) do.