ZF-3205: DisplayGroups are only rendered when FormElements decorator is used, which causes elements existent in displayGroups to render twice
When using displayGroups, elements used in displayGroups are rendered twice when $form->render() is called.
If you do not want to render each displayGroup by hand but render the form as a whole, then this goes wrong, because displayGroups only get rendered via FormElements decorator. So an element gets first rendered in a displayGroup and after that displayGroup is rendered, the element gets rendered on itself. This is no good behaviour.
I suppose that an element is cleared from the form->_elements array after a displayGroup rendered it. As a workaround, I made a displayGroup decorator who only renders displayGroups and no other elements. This applies only to my current situation of course.