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…
League-of-legends – Which Aura Items Can Be Stacked
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Related Solutions
As of patch 3.7, May 2013, the game now officially supports item sets. You can define sets of items in the launcher, and use these in-game at the shop.
You can define multiple sets, multiple categories per set, and any number of items per category.
For each item set, you can specify what champions and maps it appears on, or set them to appear globally.
In-game, you click on the Recommended tab to access a drop-down list which will have all the item sets available to that champion for that map.
If you specify an item set to be specific to a champion, it will default to that item set when you first visit the shop. Once in-game, if you select an item set through the shop, it saves that item set selection in Config/game.cfg under [RecommendedItems], and the next time you play that champion on that map it will show the item set by default.
The item sets are currently stored client-side in RADS/projects/lol_air_client/releases/*highestVersionNumber*/deploy/bin/preferences/*username*.properties. If you play on multiple computers, simply copy this file to copy your item sets.
They have plans to move them sever-side in the future, so you will eventually have access to them from any computer. They've also hinted that they may be adding import/export functionality, to share your item sets more easily.
[Updated for Season 3]
The game is currently in flux (season 3 has only been out for a couple days), but with the boosted ambient gold gain, it appears you only need one gp/10. I've been building Philo Stone to great success. If you expect to get some significant assist-gold (you're confident in your kill lane, or you're Soraka and have a good eye for your ultimate), you could probably stand to buy a Kage's Lucky Pick, but I much prefer to only buy the one Philo Stone and start immediately after into my build (Sightstone, Shurelya's, Aegis -> Bulwark).
Again though, this is all pretty fresh. Your mileage may vary.
[NOTE: The answer below was provided for season 2. There were significant changes implemented for Season 3, and this answer is now out of date. It is provided here for history, but should not be followed.]
It depends. The rule of thumb is two (Philo Stone and Heart of Gold), and 98.355% of the games, you'll want those. If the game is going to absolute crap, teamfights started early and your team is getting routed, you may have to skip the second and start pouring that extra money into wards and an oracle's (counter to what you'd think, oracle's is a great purchase if you're falling behind and the laning phase is over).
I'd be very suspicious of my support if they got a third gp/5. The only real time I can see that being worthwhile is if you know it'll be a late, late game, and that you'll need to be a significant contributor in teamfights for damage.
Best Answer
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.