I think the debuff effects like stun/paralyse/fear/silence and some other should not be applied if debuff effect already exist on target with same debuff level.
Now in pvp you can win only spam with one debuff skill like Stun(cd around 2 sec., duration 7 sec.) and you don't let it out of stun effect , because in the classic, the chance of triggering the debuff is greatly increased.