stacking bufffs/debuffs

as i understand it only item passives that are not unique will stack a debuff on and enemy player or stack buff on a teammate. is this currently true?

for instance if i see a teammate building fro heart i should go randuins or something like that... yes?

also is there a way i can find the range from the player that buffs reach? to know how far away someone can be effected by a debufff etc.