据我所知，如果我在头文件中定义一个函数，并将该头文件包含在多个源文件（翻译单元）中，并将它们链接在一起，我应该得到重复符号链接器错误。但是，我没有得到的错误有以下文件。你能解释一下我错过了什么吗？先谢了。

// include/library.hpp

#ifndef INCLUDE_LIBRARY_HPP
#define INCLUDE_LIBRARY_HPP

int add(int a, int b) {
    return a + b;
}

#endif //INCLUDE_LIBRARY_HPP

// user1.cpp

#include "include/library.hpp"

void doSomething() {
    int result = add(10, 20);
}

// user2.cpp

#include "include/library.hpp"

int main() {
    int result = add(5, 3);
    return 0;
}

# CMakeLists.txt

cmake_minimum_required(VERSION 3.25)
project(proj)

set(CMAKE_CXX_STANDARD 17)

add_library(lib INTERFACE include/library.hpp)

add_library(first_user user1.cpp)
target_link_libraries(first_user PUBLIC lib)

add_executable(second_user user2.cpp)
target_link_libraries(second_user PUBLIC lib first_user)