The demangled name for this is
ESP_WMParameter::ESP_WMParameter(char const*)
You have to take care to not ever, ever ever define functions in a header file, only to declare them. When you define a function in a header file that is included by multiple .c/.cpp files (and the error message clearly tells you that’s happening in src/main.cpp and src/WFCode.cpp), the function code will be present in all the object files resulting from these source files (main.o and WFCode.o) and the linker will then throw an error that the function was defined in multiple object files.
//.h file
#include <header_for_types_you_need.h>
// only declaration
ESP_WMParameter::ESP_WMParameter(char const*);
//.cpp file
#include <header_file_above.h>
ESP_WMParameter::ESP_WMParameter(char const*) {
/* actual function code only in .cpp file */
}
This is an extremely common beginner’s C/C++ programmer mistake and there’s at least one post per week about this type of mistake, that is. Together with the misunderstanding of the effect of #pragma once. See similiar topics like Struggling on #include directive.