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//test.h
#ifndef test_h_guard
#define test_h_guard
inline int add(int a, int b) {
    return a + b;
}
int addPlusOne(int,int);
#endif

//test.c
#include "test.h"
int addPlusOne(int a, int b) {
    return add(a, b) + 1;
}

//toast.c
#include <stdio.h>
#include "test.h"
int main(int argc, char **argv) {
    printf("%d\n", addPlusOne(2,3));
    return 0;
}

$ gcc -c -o test.o test.c
$ gcc test.o -o toast toast.c
/tmp/cc0JQ54v.o: In function `add':
toast.c:(.text+0x0): multiple definition of `add'
test.o:test.c:(.text+0x0): first defined here
collect2: ld returned 1 exit status
$