Hey,…
Instead of duplicating your geo with inversed normals,… you could also try the “twoSidedSign” node,… it basically gives you a mask for front and backfaces,… it’s often used with foliage,… quixel does it(on their new, super awesome trees) to i.e. give the leaf backfaces a different roughness than on the front faces
(you can use the two sided sign for example in a simple Lerp node as the alpha input(with maybe some more controlls before hand, to controll it’s behaviour,…))
And to your problem in generall, I’didnt do much with Lumen + Foliage so far, but what I see here looks like “raytracing AO” is on (try out different AO settings via a post process volume,…maybe this helps)
Kinda reminds me of UE4.2… The Raytracing AO was always the problem on some older and simpler Foliage 
Cheers P