Randuin's Omen Passive Stacking

So I play a lot of Kalista, and Runaan's is a core item in one of her build paths, something that I came across, that was game changing and I realised it waaay to late, was that when I was teamfighting, I felt really sluggish, I just thought it may have been packet loss or possibly a bug with attack move (since there were changes I weren't sure). Then i realised their entire team, aside from their carry had built randuin's omen, understandably. What I didn't realise, is that Runaan's Hurricane attacking multiple wearers of the Randuin's Omen unique passive stacks.. additively.

A couple of friends and I ran a custom's and tested it.

I had 1.36 Attack speed (Level 8 runes and just Runaan's blah blah) I attacked one, dropped down to 1.16 attack speed. (15% Normal, understandable) Attacked them both... 0.98. (30%?) If I had a third I'm sure it would have dropped my attack speed down to 45%.

Now, I'm not mad that I lost because of it, just mad that I realized before it was too late. The only thing I am curious is if this was intended functionality or not, if it is, that's fine, counter building that's great. I just now know for the future to switch out of Runaan's next time lol.

Edit: Stacks multiplicatively, Attacking two targets with Randuin's gave about the same AS Slow as a Randuin's Frozen Heart stack.