It's just adding math to a champion, the same as attack speed, armor, etc and appears on your stat chart during the game. It doesn't make any sense compared to all of the other unique and passive actives that appear on items. If you buy 2 kindlegems you should be able to get the cd reduction from each of them.
the weeaboo3 votesIt's to discourage you from buying too many of the same type of item. Otherwise you could just buy like 3 Warmogs, A Deadmans, A Spirit Visage, and boots. Wayyy too much health and CDR. Sure Warmogs don't stack but the passive requires 3000 health and also you only really need one of the passive.
EndlessSorcerer2 votesThat would be because they don't want players stacking a cheap CDR item (Kindlegem or Fiendish Codex) early-game.
The Sword Saint2 votesSo you are forced to finish some big items in order to stack up on certain stats instead of going for the dragonball build with 4 kindlegems, boots, and a damage item to become an obnoxious super tanky max cdr high damage threat with a tiny gold investment by the midgame.
Sona Ping1 votesCooldown Reduction is only unique on some items. Most things are just adding math to the champion, including unique passives. Like Armor Pen is just a stat but it was unique to 
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Unique passives on are there on certain items to prevent strategies that give an undue amount of power from stacking them. You mention Kindlegems; a tanky champion could get a late-game level of CDR in his build early and cheaply by buying his kindlegems first. If it wasn't unique, the Kindlegem would have to be changed such that it would make the item worse for players buying them normally, if it didn't just make the item prohibitively bad anyway.
