qt_internal_add_qml_module should warn about files being added twice

# Copyright (C) 2023 The Qt Company Ltd.
# SPDX-License-Identifier: BSD-3-Clause
set(qml_files
    "SwitchHandle.qml"
    "SwitchIndicator.qml"
    "SwitchHandle.qml"
)
qt_internal_add_qml_module(qtquickcontrols2macosstyleimplplugin
    URI "QtQuick.Controls.macOS.impl"
    VERSION "${PROJECT_VERSION}"
    PAST_MAJOR_VERSIONS 6
    CLASS_NAME QtQuickControls2macOSStyleImplPlugin
    DEPENDENCIES
        QtQuick/auto
    PLUGIN_TARGET qtquickcontrols2macosstyleimplplugin
    NO_PLUGIN_OPTIONAL
    QML_FILES
        ${qml_files}
    DEFINES
        QT_NO_CAST_FROM_ASCII
        QT_NO_CAST_TO_ASCII
)

11:58:30: Running steps for project QtDeclarative...    11:58:30: Starting: "/opt/homebrew/Cellar/cmake/3.26.0/bin/cmake" --build /Users/mitch/dev/qt-dev-debug-non-fw/qtdeclarative --target all    [1/2027 1.6/sec] Generating pc file for target Qt6::Quick    [2/2027 3.2/sec] Generating prl file for target Quick    [3/2027 3.1/sec] Running rcc for resource qmake_QtQuick    [4/2027 3.9/sec] Running rcc for resource scenegraph_shaders    [5/2027 3.9/sec] Building CXX object src/qmldom/CMakeFiles/QmlDomPrivate.dir/qqmldomfilewriter.cpp.o    [6/2027 4.1/sec] Building CXX object src/qmldom/CMakeFiles/QmlDomPrivate.dir/qqmldomindentinglinewriter.cpp.o    [7/2027 4.6/sec] Building CXX object src/quickcontrols/macos/impl/CMakeFiles/qtquickcontrols2macosstyleimplplugin.dir/.rcc/qmlcache/qtquickcontrols2macosstyleimplplugin_qmlcache_loader.cpp.o    FAILED: src/quickcontrols/macos/impl/CMakeFiles/qtquickcontrols2macosstyleimplplugin.dir/.rcc/qmlcache/qtquickcontrols2macosstyleimplplugin_qmlcache_loader.cpp.o    /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ -DGL_SILENCE_DEPRECATION -DQT_CORE_LIB -DQT_DEPRECATED_WARNINGS -DQT_DISABLE_DEPRECATED_UP_TO=0x050000 -DQT_EXPLICIT_QFILE_CONSTRUCTION_FROM_PATH -DQT_LEAN_HEADERS=1 -DQT_NETWORK_LIB -DQT_NO_AS_CONST -DQT_NO_AS_CONST=1 -DQT_NO_CAST_FROM_ASCII -DQT_NO_CAST_TO_ASCII -DQT_NO_EXCEPTIONS -DQT_NO_JAVA_STYLE_ITERATORS -DQT_NO_NARROWING_CONVERSIONS_IN_CONNECT -DQT_NO_QEXCHANGE -DQT_PLUGIN -DQT_QMLINTEGRATION_LIB -DQT_QML_LIB -DQT_STRICT_QLIST_ITERATORS -DQT_USE_QSTRINGBUILDER -DQT_WARN_DEPRECATED_UP_TO=0x070000 -D_LARGEFILE64_SOURCE -D_LARGEFILE_SOURCE -Dqtquickcontrols2macosstyleimplplugin_EXPORTS -I/Users/mitch/dev/qt-dev-debug-non-fw/qtdeclarative/src/quickcontrols/macos/impl/qtquickcontrols2macosstyleimplplugin_autogen/include -I/Users/mitch/dev/qt-dev/qtdeclarative/src/quickcontrols/macos/impl -I/Users/mitch/dev/qt-dev-debug-non-fw/qtdeclarative/src/quickcontrols/macos/impl -I/Users/mitch/dev/qt-dev-debug-non-fw/qtdeclarative/src/qml -I/Users/mitch/dev/qt-dev-debug-non-fw/qtbase/include/QtQml/6.7.0 -I/Users/mitch/dev/qt-dev-debug-non-fw/qtbase/include/QtQml/6.7.0/QtQml -I/Users/mitch/dev/qt-dev-debug-non-fw/qtbase/include/QtQml -I/Users/mitch/dev/qt-dev-debug-non-fw/qtbase/include/QtQmlIntegration -I/Users/mitch/dev/qt-dev/qtdeclarative/src/qmlintegration -I/Users/mitch/dev/qt-dev-debug-non-fw/qtdeclarative/src/qmlintegration -isystem /Users/mitch/dev/qt-dev-debug-non-fw/qtbase/include -isystem /Users/mitch/dev/qt-dev-debug-non-fw/qtbase/src/corelib -isystem /Users/mitch/dev/qt-dev-debug-non-fw/qtbase/include/QtCore/6.7.0 -isystem /Users/mitch/dev/qt-dev-debug-non-fw/qtbase/include/QtCore/6.7.0/QtCore -isystem /Users/mitch/dev/qt-dev-debug-non-fw/qtbase/include/QtCore -isystem /Users/mitch/dev/qt-dev-debug-non-fw/qtbase/mkspecs/macx-clang -isystem /Users/mitch/dev/qt-dev-debug-non-fw/qtbase/include/QtNetwork -fsanitize=address -fno-omit-frame-pointer -fno-optimize-sibling-calls -g -std=c++17 -arch arm64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX13.3.sdk -mmacosx-version-min=11.0 -fPIC -fvisibility=hidden -fvisibility-inlines-hidden -Wall -Wextra -fno-exceptions -Werror "-Wno-error=#warnings" -Wno-error=deprecated-declarations -fapplication-extension -MD -MT src/quickcontrols/macos/impl/CMakeFiles/qtquickcontrols2macosstyleimplplugin.dir/.rcc/qmlcache/qtquickcontrols2macosstyleimplplugin_qmlcache_loader.cpp.o -MF src/quickcontrols/macos/impl/CMakeFiles/qtquickcontrols2macosstyleimplplugin.dir/.rcc/qmlcache/qtquickcontrols2macosstyleimplplugin_qmlcache_loader.cpp.o.d -o src/quickcontrols/macos/impl/CMakeFiles/qtquickcontrols2macosstyleimplplugin.dir/.rcc/qmlcache/qtquickcontrols2macosstyleimplplugin_qmlcache_loader.cpp.o -c /Users/mitch/dev/qt-dev-debug-non-fw/qtdeclarative/src/quickcontrols/macos/impl/.rcc/qmlcache/qtquickcontrols2macosstyleimplplugin_qmlcache_loader.cpp    /Users/mitch/dev/qt-dev-debug-non-fw/qtdeclarative/src/quickcontrols/macos/impl/.rcc/qmlcache/qtquickcontrols2macosstyleimplplugin_qmlcache_loader.cpp:25:38: error: redefinition of 'unit'        const QQmlPrivate::CachedQmlUnit unit = {                                         ^    /Users/mitch/dev/qt-dev-debug-non-fw/qtdeclarative/src/quickcontrols/macos/impl/.rcc/qmlcache/qtquickcontrols2macosstyleimplplugin_qmlcache_loader.cpp:11:38: note: previous definition is here        const QQmlPrivate::CachedQmlUnit unit = {                                         ^    1 error generated.    [8/2027 3.8/sec] Building CXX object src/qmldom/CMakeFiles/QmlDomPrivate.dir/qqmldomlinewriter.cpp.o    [9/2027 2.6/sec] Building CXX object src/qmldom/CMakeFiles/QmlDomPrivate.dir/qqmldomerrormessage.cpp.o    [10/2027 2.4/sec] Running AUTOMOC file extraction for target Quick    [11/2027 2.4/sec] Building CXX object src/qmldom/CMakeFiles/QmlDomPrivate.dir/qqmldomfieldfilter.cpp.o    [12/2027 2.2/sec] Building CXX object src/qmldom/CMakeFiles/QmlDomPrivate.dir/qqmldommock.cpp.o    [13/2027 1.9/sec] Building CXX object src/qmldom/CMakeFiles/QmlDomPrivate.dir/qqmldommoduleindex.cpp.o    [14/2027 1.6/sec] Building CXX object src/qmldom/CMakeFiles/QmlDomPrivate.dir/qqmldomexternalitems.cpp.o    [15/2027 1.4/sec] Building CXX object src/qmldom/CMakeFiles/QmlDomPrivate.dir/qqmldomelements.cpp.o    [16/2027 1.4/sec] Building CXX object src/qmldom/CMakeFiles/QmlDomPrivate.dir/qqmldomitem.cpp.o    ninja: build stopped: subcommand failed.    11:58:42: The process "/opt/homebrew/Cellar/cmake/3.26.0/bin/cmake" exited with code 1.    Error while building/deploying project QtDeclarative (kit: qt-dev-debug-non-fw)    When executing step "Build"    11:58:42: Elapsed time: 00:11.