when they make the items unable to stack.
They are making it so you can't get 4
or
and get 60% extra healing and shields. As they are not unique named passives(like icy) they can stack from different items, but extra copies of the same one don't.
(+20% cooldown unique passive) and
(20% cooldown passive) give 40% cdr. But two
or
would only give 20%.
So basicly you can get both Ardent and Crucible for 30%,(forbidden idol for 40% but that would be better served for a straight up ap item) without having to choose.
From the Wiki
Unique is a term used to describe an item effect. Stacking items—using more than one of the same item— will not result in multiple applications of the item effects that are labelled as unique.
Unique passive effects only apply once from identical items.
Unique active effects of identical items do not stack and share the same cooldown.
Unique aura effects apply once to yourself and once to allies, so two champions with the same unique aura item will each benefit twice from the aura, but other champions without the item will only be affected once.
From Surrender@20
Ardent Censer
+15% heal and Shield power now a Unique Passive
Mikael's Crucible
+15% heal and Shield power now a Unique Passive
And on the diffrence between named effects and unique effects from the wiki
Named item effects function similarly to unique effects, except that the effect is unique across any and all items which provide the named effect. Similar effects that do not share a name will stack, such as Quicksilver and Mikael's Crucible.