Say I'm playing Irelia mid and I wanna grab mercs for the MR vs a mage, and I take grasp of the undying so I have access to the extra tenacity in the mastery tree, what's my max tenacity? Or is it all unique and I've been wasting resources all this time?
DrCyanide2 votesLets say you've got a 10 second stun (for easy numbers)
= 30% tenacity = 10 seconds * (1 - 0.3) = 7 second stun
+ 
= 30% + 25% tenacity = 10 seconds * (1 - 0.3) * (1 - 0.25) = 5.25 second stun
So despite having more than than 50% tenacity you don't reduce CC by 50%.
The Bearded Bard1 votesOkay so the value diminishes based on each individual source, assuming the highest value is treated as the first source, then down from there.
DrCyanide1 votesIt's multiplication, the order doesn't matter.
Critmaster Garen1 votesthe only confirmed instance of tenacity stacking additive is + 
to 60 % for 8 seconds when the shield is triggered.
otherwise, tenacity always stacks multiplicative.