QWidget::closeEvent is called twice (it doesn't matter if the event is accepted or not).
See attached minimal application.
This is a regression against Qt 5.2.
For some reason Qt Creator still only got a single closeEvent call, until the patch for fixing
QTBUG-37933 was integrated - since then Qt Creator also gets the closeEvent twice, leading to a bunch of issues e.g. regarding saving and restoring sessions.