$ cat a.c
#include "b.c"

static int foo = 42;

int main(int argc, char ** argv)
{
    foo=argc;   // dummy code
    return foo; // dummy code
}
$ cat b.c
static int foo = 42;
$ cc -Wall -pedantic a.c
a.c:3:12: error: redefinition of ‘foo’
    3 | static int foo = 42;
      |            ^~~
In file included from a.c:1:
b.c:1:12: note: previous definition of ‘foo’ with type ‘int’
    1 | static int foo = 42;
      |            ^~~
b.c:1:12: warning: ‘foo’ defined but not used [-Wunused-variable]
$