QDomAttr QDomElement::setAttributeNode ( const QDomAttr & newAttr ) does NOT replaces attribute with the same name as newAttr.

Manual says
"QDomAttr QDomElement::setAttributeNode ( const QDomAttr & newAttr )
Adds the attribute newAttr to this element.
If the element has another attribute that has the same name as newAttr, this function replaces that attribute and returns it; otherwise the function returns a null attribute.",

but it seems it adds newAttr regardless of earlier presence of an attribute with the same name - after using it referenced QDomElement has two attributes with the same name and different values.