Newb here, Making up small numbers for easy math,
But can you double dip on runes that add +% dmg to mob when attack
For Example, lets say rend does 100 dmg i put mutilate rune on it, so it lets all mobs take +10% more dmg for the 5 seconds its ticking. I rend once, it does 100dmg and gives it the debuff, if i re-rend will it now overrite the old rend with 110dmg since there debuffed for 10% more damage?
For most effects (like Rend: Mutilate). Those debuffs stack only a single time. So, in your example, if you hit a guy with Rend, the first hit will do 100 damage, the second 110, the third 110, the fourth 110, etc.
Also, the 10% you get from Mutilate is additive, and goes into a big pile with lots of other stuff, like Rend% on armor, the Taeguk gem, the Berserker Rage and Brawler passives, and lots of other stuff (you can take a look here). So in reality, you’ll be getting way less than 10% extra damage from that rune.