For example, if I DW’d 2x black flame daggers, will FoI debuff enemies for 36% damage reduction?
This thread would indicate no…
http://www.grimdawn.com/forums/showthread.php?t=31756
ETA:
Actually, after reading more on it, I’m not sure either. My question is, does that particular debuff follow the same syntax as RR?
But these are buffs to a skill which in my head should occur before any of the game mechanics apply.
Huh this is the first question in a while that I’m not sure on. The damage will definitely stack but this… it should, I suppose. But I have never read anything about it. Does it stack like -X% resist reduction, or like damage absorption, or not at all? That thread from 2016 is interesting but obviously item skill modifiers didn’t exist back then.
Edit: in my experience all other modifiers simply stack, like cooldown reduction (various weapons with this stat dual wielded work just fine). I don’t see why this damage reduction wouldn’t but who really knows. Not me, anyway.
In this case debaffs have same duration so answer is yes, they stuck. It’s similar to Plaguebearer’s Master’s Spellblade.
They do stack, because they add something to the skill. It’s like putting food to your fridge, you don’t put meat there and then take it off right after you’ve put it to put salad there.
So basically, it’s skill + modifier + modifier in that situation. 36% damage reduction it will be.
Same would be with Mythical Dreadweaver - 40 reduction to all resists. Don’t know if in these cases the duration is increased too, grimtools says yes, but that’s about it. Someone would need to try it out tbh. 
Don’t look at me. I don’t have time to test stuff now.
Duration doesn’t stuck, it’s a common issue in grimtools, Dammit promised to fix this.
I don’t think damage reduction stacks, and here’s why:
On the left is an in-game screenshot which shows that flat resist reduction doesn’t stack. On the right is a screenshot of Grimtools which makes it look as though it DOES stack. Even though looking at the skill modifiers in Grimtools for Flames of Ignaffar while wearing two black flame daggers will show 36% damage reduction, that doesn’t mean that it works the same in-game. Always default to what the game is telling you rather than what a 3rd party tool tells you.
Damage reduction (just like flat resist reduction shown above) never stacks in Grim Dawn for any reason whatsoever, so I don’t see why skill modifiers would be an exception.
[redacted, see following post]
These debaffs have different duration, so they work separately and don’t stack.
But as I said in my 1st message same duration stacks.
You’re talking about different case, man. Flat debuffs from item itself and it’s affix counts as the same source.
I checked out the master’s spellblade and it looks like the flat rr does indeed stack from the affix.
I double-checked with a “tyrant’s [insert item here] of the abyss” and the damage reduction stacks too. My bad, my initial test was based on debuffs of different durations, so it messed up the conclusion I made.
If item modifiers work like affixes do, then they should stack just fine as long as they’re modifying the same skill and they’re of the same duration
Hmmmmm, tyrant’s of the abyss looks like the hero nightblade mastery need. RoS can become an AoE damage debuff…
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Thanks for the clarification guys. Makes sense in my head for them to anyhow. 
Regardless, this should open up some interesting combinations.
Dual wielding chaos daggers with one class being inquisitor (let’s forget about that modifier to AAR, better for everyone), interesting… but niche build if you ask me.