Agree, only accessories seem to stack if dupicated and, then not if the actual item name is duplicated. Two fade touched that give the exact same, even if different materials will not stack but say one gives 5%, the other 10%, then you do get 15% but, I suspect 99% is max so, 100% = 0% in DAI math (or frostbite math if you prefer.)
Non stacking is the general RoT (rule of thumb) for RPG type games so, yes you should assume things do not stack on a single character unless you have proof, either others know it does or, you see it stack yourself. Now party wide, sure two party members can have the same bonus or proc and, that will meant that procs more often for your group, but one per character.
Say you really love Veil Strike, craft all armor with Fade Touched Lurker Scales, that's 10% chance of casting Veilstrike on a hit. All four of your paty has it so, yes the enemy is going to pretty much end up glued to the ground. Fun and, pretty useful against fools that want to rush my mage LOL. Add in Solas and, be a rift mage yourself and, well, they can't get up, if someone's armor doesn't Veilstrike fast enough, you or Solas can. (AKA how to deal with FBB and Decent at level 15 on nightmare and take less than 10% damage. LOL)