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. Here is as far back as I can find relevant information about the subject so far concerning PVE.
http://www.elitegamerslounge.com/hom...?f=164&t=56604
The bottom poster says what I remember.