Can referencing the library with TWO includes cause problem?

This is probably a linking error. When you compile the client program, you need to tell the linker where to find the library. For linux that would be

gcc client.c -L<path/to/lib> -llib -o client

. You can find instructions for doing that for a VisualStudio project Here[^]

Your question asks "Can referencing the library with TWO includes cause problem?". The answer is "Maybe". If the include file has #include guards, then no, but otherwise, "Probably".
example #include file with guards

#ifndef MY_LIB_H
#define MY_LIB_H

struct s {
   int i;
    double d;
};    

int foo(struct s  *my_struct);

#endif

Without the include guards, the compiler should complain about multiple definitions of struct s, if the header file is included more than once.