Duplicate symbol while invoking headers from mainwindow.h

Question

Building the following cmake project from the trivial Qt6 project below gives me the following linker error when I #include "utils.hpp" from mainwindow.h:

duplicate symbol 'get_test_map()' in:
    CMakeFiles/qt_minimal.dir/qt_minimal_autogen/mocs_compilation.cpp.o
    CMakeFiles/qt_minimal.dir/src/gui/main.cpp.o
duplicate symbol 'get_test_map()' in:
    CMakeFiles/qt_minimal.dir/qt_minimal_autogen/mocs_compilation.cpp.o
    CMakeFiles/qt_minimal.dir/src/gui/mainwindow.cpp.o
ld: 2 duplicate symbols for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

According to an answer to a similar question, having the #include "utils.hpp" in mainwindow.cpp instead solves the linker error. However, I would like the class MainWindow to call functions from external headers which requires having the include in MainWindow.h itself.

How do I fix this issue?

CMakeLists.txt

cmake_minimum_required(VERSION 3.24)
project(qt_minimal)

set(CMAKE_CXX_STANDARD 20)

find_package(Qt6 COMPONENTS Widgets REQUIRED)
set(CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake)
include(QtCommon)

set(GUI_SOURCE_FILES
        main.cpp
        mainwindow.cpp
        )

add_executable(${PROJECT_NAME} ${OS_BUNDLE} # Expands to WIN32 or MACOS_BUNDLE depending on OS
        ${GUI_SOURCE_FILES} ${META_FILES_TO_INCLUDE} ${RESOURCE_FILES}
        )

target_precompile_headers(${PROJECT_NAME} INTERFACE QtWidgets.h)

target_link_libraries(${PROJECT_NAME} Qt6::Widgets)

main.cpp

#include <QApplication>
#include "mainwindow.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    MainWindow mainWindow;
    mainWindow.show();
    return app.exec();
}

utils.hpp

#ifndef QT_PROJECT_GUI_UTILS_HPP
#define QT_PROJECT_GUI_UTILS_HPP

#include <unordered_map>
using Map = std::unordered_map<std::string, std::string>;

Map get_test_map() {
    Map m;
    m["accountID"] = "LKH7876";
    m["BankBalance"] = "1,00,000";
    m["LoanAmt"] = "10,00,000";
    return m;
}

#endif

mainwindow.h

#ifndef mainwindow_h
#define mainwindow_h

#include <QMainWindow>
#include "utils.hpp" // <-- Offending include

namespace Ui
{
    class MainWindow;
}

class MainWindow : public QMainWindow {
    Q_OBJECT

public:
    MainWindow(QWidget *parent = 0);
    virtual ~MainWindow();

private:
    QScopedPointer<Ui::MainWindow> ui;
};

#endif

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
        : QMainWindow(parent),
          ui(new Ui::MainWindow) {
    ui->setupUi(this);
    // do something with get_test_map()
}

MainWindow::~MainWindow() = default;

QTCommon.cmake

macro(fix_project_version)
if (NOT PROJECT_VERSION_PATCH)
    set(PROJECT_VERSION_PATCH 0)
endif()

if (NOT PROJECT_VERSION_TWEAK)
    set(PROJECT_VERSION_TWEAK 0)
endif()
endmacro()

macro(add_project_meta FILES_TO_INCLUDE)
if (NOT RESOURCE_FOLDER)
    set(RESOURCE_FOLDER src/gui/res)
endif()

if (NOT ICON_NAME)
    set(ICON_NAME AppIcon)
endif()

if (APPLE)
    set(ICON_FILE ${RESOURCE_FOLDER}/${ICON_NAME}.icns)
elseif (WIN32)
    set(ICON_FILE ${RESOURCE_FOLDER}/${ICON_NAME}.ico)
endif()

if (WIN32)
    configure_file("${PROJECT_SOURCE_DIR}/cmake/windows_metafile.rc.in"
      "windows_metafile.rc"
    )
    set(RES_FILES "windows_metafile.rc")
    set(CMAKE_RC_COMPILER_INIT windres)
    ENABLE_LANGUAGE(RC)
    SET(CMAKE_RC_COMPILE_OBJECT "<CMAKE_RC_COMPILER> <FLAGS> -O coff <DEFINES> -i <SOURCE> -o <OBJECT>")
endif()

if (APPLE)
    set_source_files_properties(${ICON_FILE} PROPERTIES MACOSX_PACKAGE_LOCATION Resources)

    # Identify MacOS bundle
    set(MACOSX_BUNDLE_BUNDLE_NAME ${PROJECT_NAME})
    set(MACOSX_BUNDLE_BUNDLE_VERSION ${PROJECT_VERSION})
    set(MACOSX_BUNDLE_LONG_VERSION_STRING ${PROJECT_VERSION})
    set(MACOSX_BUNDLE_SHORT_VERSION_STRING "${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}")
    set(MACOSX_BUNDLE_COPYRIGHT ${COPYRIGHT})
    set(MACOSX_BUNDLE_GUI_IDENTIFIER ${IDENTIFIER})
    set(MACOSX_BUNDLE_ICON_FILE ${ICON_NAME})
endif()

if (APPLE)
    set(${FILES_TO_INCLUDE} ${ICON_FILE})
elseif (WIN32)
    set(${FILES_TO_INCLUDE} ${RES_FILES})
endif()
endmacro()

macro(init_os_bundle)
if (APPLE)
    set(OS_BUNDLE MACOSX_BUNDLE)
elseif (WIN32)
    set(OS_BUNDLE WIN32)
endif()
endmacro()

macro(fix_win_compiler)
if (MSVC)
    set_target_properties(${PROJECT_NAME} PROPERTIES
        WIN32_EXECUTABLE YES
        LINK_FLAGS "/ENTRY:mainCRTStartup"
    )
endif()
endmacro()

macro(init_qt)
# Let's do the CMake job for us
set(CMAKE_AUTOMOC ON) # For meta object compiler
set(CMAKE_AUTORCC ON) # Resource files
set(CMAKE_AUTOUIC ON) # UI files
endmacro()

init_os_bundle()
init_qt()
fix_win_compiler()