When you add an object of Zend_Form_DisplayGroup to the Zend_Form it will not remove the elements in Zend_Form. When rendering the form, it will render the elements twice. (in the decorator FormElements of Zend_Form and the in the decorator for the Zend_Form_DisplayGroup)

testcase: (it will render the element test twice)

$oForm = new Zend_Form();
$oForm->addElement('text','test');
        
$oDisplayGroup = new Zend_Form_DisplayGroup('bug',$oForm->getPLuginLoader('decorator'));
$oDisplayGroup->addElement($oForm->getElement('test'));
$oForm->addDisplayGroups(array($oDisplayGroup));
echo $oForm;

solution: add following code in _addDisplayGroupObject in Zend_Form (it will remove the elements from the ordering in Zend_Form, so they won't be rendered)

foreach ($group->getElements() as $element) {
    unset($this->_order[$element->getName()]);
}