Description:
The 'libretro' group includes the individual libretro shader packages (libretro-shaders-cg, libretro-shaders-glsl, libretro-shaders-slang), each of which both provide and conflict with libretro-shaders thus creating a conflict when one attempts to install the group itself.

Steps to reproduce:
pacman -S libretro

I'm not sure if this is just a case of groups Working As Intended since the only way around it would be to either remove the shaders from the group or make libretro-shaders depend on another package (it's already an optdepend of retroarch).