VRay Environment Fog weaker when duplicated

[Furnace]

Hi,
I'm trying to make a car tunnel, I've got the light's looking just about right in a single section of the tunnel (01) but when I duplicate it they look completely different, all detail of the volume light/fog fades into a haze.

Would anyone know the cause of this - what I need to amend in the light setting or fog settings? I'm on a laptop so struggling to do too many test renders.

Many thanks,
Elliot

[Art48]

There is some setting about opacity fade in gizmos or sth in there which you can set to multiply and something else. maybe its overlaying each other and thus multiplying it will darken/lessen the effect.