Multi threaded function already defined in .obj

I've searched for the error LNK2005 "already defined in .obj" but can't find content related to the specific problem I am facing. Hope someone can help me on this...

I've a header foo.h

// foo.h

#ifndef FOO_H
#define FOO_H

void foo() {
    print("foo\n");
}

#endif

and main file... main.cpp

// main.cpp

#include <thread>
#include "foo.h"

int main() {

    std::thread t(foo);
    t.join();

    return 0;
}

Now, it compile without any errors and gives the gives output to the console...

foo

But if I create another file foo.cpp and just include the header foo.h and do nothing else...

// foo.cpp

#include "foo.h"

...I get linker error LNK2005 "void __cdecl foo(void)" (?foo@@YAXXZ) already defined in main.obj

Don't know what's going wrong here.?!!