You and nearby allies deal x% increased damage and similar mechanics doesn't stack

Hello,

"You and nearby allies deal x% increased damage" - synthesis implicit
"Nearby allies gain 18% increased damage" - Dying Breath Iron Staff

Those 2 modifiers are overwritten by each other, if gained from multiple sources like: other players Blink Arrow, Animated Guardian, Holy Armament created by Holy Strike, Animate Weapon of Self Reflection etc. And behave like auras, only the strongest one apply(maybe do to pre delirium league most buffs where implemented as an aura).

While "You and nearby allies gain 50% increased damage" granted by Leer Cast Festival Mask indeed do stack.