There are certain items in-game that you can stack, like WotA for example. What other items can also be stacked? Curious, because I've mistakenly bought Aegis when somebody else had...
Aegis of the Legion can stack like Will of the Ancients does. The way WotA stacks is it'll stack only between the people who have the items, so people around you won't get double the spell vamp. Only the two people will have the double aura.
This is also the same with Aegis of the Legion. So if Amumu had an Aegis he would get the aura. If Galio also had an Aegis, Amumu would get the bonus from Galio's aura and Galio would get the bonus from Amumu's aura, even though they both already have Aegis. However, the Malzahar who didn't buy an Aegis would not have both the auras.
As of May 2013, items that grant Auras: Emblem of Valor, Banner of Command, Aegis of the Legion, Runic Bulwark, Mana Manipulator, Shard of True Ice, Zeke's Herald, Abyssal Scepter, Frozen Heart, Will of the Ancients
Abyssal Scepter and Frozen Heart are offensive auras, and debuff enemies in range. Multiple champions with this item debuff the enemy once.
As for the rest of the items, assuming all 5 champions are next to each other:
It can sometimes be beneficial to have multiple aura items, particularly when your team is spread out. Mouse over the item to see its aura range. Some are quite small, Abyssal Scepter in particular (its meant more for melee AP champions than ranged).
Unique Aura Names
The unique keyword is not limited to auras, and it indicates that the benefits tagged as unique can only apply once. The intent is to prevent players from buying the same item multiple times to receive the unique benefits more than once. With auras, the stacking rules given above apply, but with one small wrinkle.
Items with the same unique aura name do not stack if you have multiple items with the same Unique aura name in them. This is usually seen when an aura item is an upgrade component of another item (Aegis of the Legion & Runic Bulwark, Mana Manipulator & Shard of True Ice, Emblem of Valor & Banner of Command). The upgrade component has a similar aura, but less powerful than the final item. They both have a "Unique aura - MatchingNameHere" property.
For example, if one champion has an Aegis of the Legion, and another has a Runic Bulwark, both items have the "Unique aura - Legion" keyword, but the benefits of the aura differ on each item. The other three champions will receive the more powerful Runic Bulwark aura, and the two aura champions will receive both the Aegis of the Legion aura and the Runic Bulwark Aura.
Any item that doesn't say UNIQUE passive on it when it says just passive it can be stacked although certain items like will of the ancients stack if two different people buy it so people use double wota anything else with UNIQUE is not stackable