Just so you know:
“However if you then hook up also another parameter in the material in the exposed inputfunction input, (which overrides the parameter you have placed in the function into the functionInput node) both the parameter in the function and the one placed in the material will now be exposed in the instance material.”
If you care to write the exact same name (and i guess also the Group), Unreal replaces the original Input so it only shows one parameter.