After the new patch my stuns will not override a previous stun. It will wait until it wears off before a new stun can be applied, even though it lands.
Ok from what I remember, stun always would land and override the previous stun. As in, a 2 sec would override an 8 sec stun and vice versa. The only time I would see a stun immunity was in PVP, there was a small timer that you could not be stunned again.
As far back as I can find relevant information about the subject so far concerning PVE, I present this link.
http://www.elitegamerslounge.com/hom...?f=164&t=56604
The bottom poster says what I remember.