This error would occur if you write a global variable definition in your header like
#ifndef _MAIN_H_
#define _MAIN_H_
const char user_PW[] = "ABC";
#endif
If multiple .c/.cpp files include this header, each of them will attempt the global variable, however, there can only be one instance of it. The correct way is to use an extern declaration in the header, and a definition of the variable in one .cpp file. This has been discussed multiple times in topics like e.g. Splitting cpp files - #2 by maxgerhardt.
In which file do you define this variable?