I might have just been seeing my blade of venom doing some strikes on the mob.I have tried your experiment and have been timing when I should see the damage show up on the mobs head and it doesn't happen, and there is no double hit either. I think I am right on this one.I have to point out, Your I am 95% sure that when you cast frenzy, your auto is not interrupted.
Try it and watch your damages aboe the mob.
You will have a steady flow of damage, say 1 per second, and then cast frenzy and no damage is done, once it is cast, 2-3 damage amounts pop up all in one go above the mob.
Due to this, I think that it does not interrupt your auto attack.
Either way. The skill is way too short. 60 sec plus please.
I will try it with the offhand un equipped next time