Here is a quickie
From the Shaman..
Spoiler
Show
lvl 1 Protector Spirit Shaman feature. Bear Shaman Spirit boon: Lets allies gain additional HP when you heal them next to spirit ally.
lvl 11 Scarred Healer Paragon Path. Scar's gift: Lets allies gain additional HP when you heal them next to spirit ally.
Both are based on Con Mod, do these two class features stack?
Errr I tried not to go into copyright infringement so it may be a bit vague... haha but I see this as stacking "class features" which was somewhat common in 3.5 where someone would get the same attribute to the same effect.. Like getting Cha to melee damage from multiple sources... Like having a Killoren Paladin using the racial smite and the paladin's smite... (not sure if those both stack or not but it is just an example... You can find a list somewhere on these here forums).