Does Item stacking work or get reduced?
like 2
and such, I assume its reduced because otherwise ap champs would just build a lot of 
like 2
and such, I assume its reduced because otherwise ap champs would just build a lot of 
You cant stack rabbadons, most item effects cant be duplicated by purchasing the same item. Sometimes different items with the same effects cant have the effect stacked
. You have to read wether something is a passive or a unique passive.
so is it essentially an empty slot? or just one with the ability power without the unique passive?
You get 120 ap from both, but you only get one application of the passive +35%.
So you're getting 240 ap + 35% ap if you use 2 raba, instead of 240 + 35% + 35% (not +70%, it would apply twice)
so Im assuming it would be better to just get a different item then
When an item has a "Unique passive" they cannot be stacked, meaning you won't get the effect of that Unique passive more than once. If they just have a regular Passive effect then you can stack them, hence why people used to stack Doran's rings before their passive was changed into a Unique.