Was making some quick tests regarding Frenzy, easy to do just looking at your own attack speed in the character sheet. I expected that either it didn’t stack at all, or that it always stacked (from different sources) - but the result was that some stack, some don’t, which feels under-documented in the tooltips.
Frenzy from Howl and Frenzy Totem override each other. Even if one is weaker than the other, only one will be active (couldn’t figure out the priority, sometimes it was one, sometimes it was the other, independent of the order they were applied).
But Frenzy from Leap Attack stack with the Frenzy from Howl or Frenzy Totem. They each have their own independent duration (despite the buff bar not showing that), and while both are active you have higher attack speed and any other buffs from them are both applied.