Do spells stack?

I have an eternal hailfire and a cataclysm on a wizard. When i cast a storm, and then another storm procs from barrage, the first is cancelled. but when twister procs, and other procs from gear(cataclysm and other stuff), i see multiples of them.Is this a bug? Or is that just the way it is? And is it the same with all wizard spells, proc or or self cast?

Per 2.0 patch notes: