clipping_planes_pars_vertex.glsl uses this coding pattern

#if NUM_CLIPPING_PLANES > 0 && ! defined( PHYSICAL ) && ! defined( PHONG )

    varying vec3 vViewPosition;

#endif

In other words, declare viewViewPosition -- but not if it has been predeclared.

These material-dependent chunks become problematic when creating new materials, because it can lead to variable redeclaration conflicts if it is not known if the variable has been previously declared. For example:

#if SOME_CONDITION

    varying vec3 vViewPosition;

#endif

...

//from clipping_planes_pars_vertex
#if NUM_CLIPPING_PLANES > 0 && ! defined( PHYSICAL ) && ! defined( PHONG )

    varying vec3 vViewPosition;

#endif

As the number of materials increases, figuring out how to modify the chunks to avoid a potential redeclaration is too tedious.

Would the following coding pattern be acceptable? To be used only when needed.

varying vec3 vViewPosition;
#define VVIEWPOSITION

...

//rewrite of clipping_planes_pars_vertex
#if NUM_CLIPPING_PLANES > 0 && ! defined( VVIEWPOSITION ) 

    varying vec3 vViewPosition;

#endif

Or, is there a better solution to decouple the material types from the Chunks?