As in the title - unique passives are not supposed to stack but they seem to do in this case (maybe for the 20% of Nashor's Tooth it's intended, anyways happens also with two 10% items).
Edit: I didn't see in the rules when it says that non-named passives can stack for different items - not a bug!
Deep Terror Nami2 votesIf it says "Unique", that means it's unique between two items of the same name (I.E. two glacial shrouds), but not between different items (I.E. glacial shroud and fiendish codex).
Check out the LoL wiki page for in-depth descriptions http://leagueoflegends.wikia.com/wiki/Unique
GamerThink1 votesOk then it's my mistake... good to know that, I always played like it didn't... XD
So the only point of these passives being unique is not having a 
AP marksman I guess... :P
Deep Terror Nami1 votesWell, those would still be terrible, but yes. I think the only item ever worth building two of is Phantom Dancer, since it's purchased for the stats rather than the passive ability.
GamerThink1 votesHuh terrible really? With 6 Nashor you would make around +540 magic damage on hit with +240% AS (and 40% cdr for the spells, if that even counts)
ValyrianBlade1 votesWhich items specifically? Or are you saying any of them? E.g. 2 glacial shrouds?
GamerThink1 votesI just tried Glacial Shroud + Fiendish Codex. Don't know if it's for all combinations. Also I know that Nashor + Glacial Shroud stack.
Zombiemaster1 votesIf a Unique passive doesn't have a name, then it will stack only if it's from different items. So, the CDR unique passive will stack from those items since they are different, however if you had 2 Glacial Shrouds or 2 Fiendish Codex's then it wouldn't stack.