The rules is not "all increased modifiers are additive with each other" - as a trivial example, increases to armour are not additive with increases to, say, fire damage, because they are applying to different things. The rule is that all "increased" (and "reduced") modifiers to the same value are additive with each other.
The damage of a weapon item, and the damage of a player using that weapon, are not the same value.
A local stat granting increased damage to a weapon is addtive with other modifiers to the damage of the weapon - multiple such modifiers on the same item stack additively. They are all affecting the base physical damage the weapon has from it's base type. The final result of applying all such modifiers provides the total damage of the weapon, which is displayed above the mods (as noted elsewhere in the thread, if a weapon's physical damage is modified it's value will be displayed in blue to show this, while elemental and chaos damag...
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