qmake does not check for duplicate files in .pro file

(tested with qmake 3.1)

Starting point: a .pro file with one .cpp and one .h file listed twice (please refer to QTCREATORBUG-18926):

• Invoke qmake and make from Qt Creator, i.e. build the project (command lines are stripped of paths)
• qmake.exe -o Makefile InterlockingConfigurator.pro -spec win32-g++ "CONFIG+=debug" "CONFIG+=qml_debug"
• mingw32-make -f Makefile.Debug
• Make complains about duplicate recipes for moc_theclass.cpp and moc_theclass.o
• Linker complains about duplicate symbols (like ui_theclass.h:81: error: multiple definition of `TheClass::TheClass(QWidget)'*)

QMake should detect duplicate names and only generate Make rules for one occurrence or should stop with error or warning.