For example, If I play against 5 Cassiopeias, will the 20% damage reduction apply to every single twin fang/ poison tick I'm hit with across each player? Is it technically considered the same spell? Or will it only apply the reduction on an individual basis?
NekoniClaws1 votesAll of them count as the same spell.
https://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2838906474/234492135?tab=overview
^I fed my ass off this game, this is not a bragging link. You can tab to player builds and see where we realized 
might work, and you can compare that to the timeline graph. You can just imagine how many times 5 brand ults bounced onto 5 divers that were all in the same tiny part of the lane.
