you cant use the same item twice. so for example you couldn't use that stun item twice -> no permanent stun 
but i think that dodge will be sumed. maybe there is a maximum dodge of 30% like in dota... if im right it was 30%... but critical doesn't sum directly i think. then you have 2 chances for critical, 1x 5% for double damage and a second time 5% for double damage.what exactly means you got something about 7,5% because they also can trigger at the same time, what means one of them gets overwritten in my opinion.
a better example is the possibility of having this:
1x 10% chance for 1,5 times normal damage and
1x 5% chance for 2 times normal damage
those criticals couldn't sum together, and most time in game the items got different stats in point of crit and chance.
EDIT: additionally i don't think that possible skill-releated crit skills would be summed up with item-releated crits 
what I'm interested in is if the bonus damage of items and so on is added to damage you deal before the crit damage is calculated or after that,
so that 100+50 damage will deal 250 damage with doubledamage crit or deal 300 damage...?