CUnit Tests getting Build Error

Its my first time testing C programs. I have this header file which I would like to test:

#ifndef CALCULATOR_HELPER_H
#define CALCULATOR_HELPER_H
#endif

    int add(int num1, int num2) {
        return num1 + num2;
    }

I am using the framework CUnit to test it. I am using Netbeans as an IDE. The following is the code.

#include <stdio.h>
#include <stdlib.h>
#include "CUnit/Basic.h"
#include "calculator_helper.h"

/*
 * CUnit Test Suite
 */

int init_suite(void) {
    return 0;
}

int clean_suite(void) {
    return 0;
}

/* IMPORTANT PART: */

void testAdd() {
    int num1 = 2;
    int num2 = 2;
    int result = add(num1, num2);
    if (result == 4) {
        CU_ASSERT(0);
    }
}

int main() {
    CU_pSuite pSuite = NULL;

    /* Initialize the CUnit test registry */
    if (CUE_SUCCESS != CU_initialize_registry())
        return CU_get_error();

    /* Add a suite to the registry */
    pSuite = CU_add_suite("newcunittest", init_suite, clean_suite);
    if (NULL == pSuite) {
        CU_cleanup_registry();
        return CU_get_error();
    }

    /* Add the tests to the suite */
    if ((NULL == CU_add_test(pSuite, "testAdd", testAdd))) {
        CU_cleanup_registry();
        return CU_get_error();
    }

    /* Run all tests using the CUnit Basic interface */
    CU_basic_set_mode(CU_BRM_VERBOSE);
    CU_basic_run_tests();
    CU_cleanup_registry();
    return CU_get_error();
}

PROBLEM

When I am building the test, I am getting a BUILD TESTS FAILED. More specifically, I get this:

In function `add': NetBeans/Calculator/calculator_helper.h:12:
multiple definition of `add'
build/Debug/GNU-Linux-x86/tests/tests/newcunittest.o:NetBeans/Calculator/./calculator_helper.h:12:
first defined here collect2: error: ld returned 1 exit status

Can anybody tell me why I am getting this error. I tried searching on google but I found no luck.

Solution

I have this header file which I would like to test:

You're defining a function in a header file:

int add(int num1, int num2) {
    return num1 + num2;
}

Declare it in the header:

#ifndef CALCULATOR_HELPER_H
#define CALCULATOR_HELPER_H

int add(int num1, int num2);

#endif      /* the endif goes at the end of the file */

...and define it in a source file:

#include "helper.h"

int add(int num1, int num2) {
    return num1 + num2;
}