How can I avoid ambiguous call to parent functions with custom QGraphicsItem?

Question

Let's suppose that there is a scene with several custom QGraphicsItem's. And each such item has ability to scale (I place special anchors to the corners). It work good, as expected.

Now I want to group some of items with QGraphicsItemGroup and I would like to add same scale ability into group item.

Below is an example of my code:

class Base : public QGraphicsItem
{
public:
Base(QGraphicsItem * parent = 0);
...
// I implement scaling with these functions:
void mouseMoveEvent(QGraphicsSceneMouseEvent * event);  
void mousePressEvent(QGraphicsSceneMouseEvent * event);  
void mouseReleaseEvent(QGraphicsSceneMouseEvent * event); 
}

class Item : public Base
{
public:
    Item(QGraphicsItem * parent = 0);
    QRectF boundingRect() const;
    void paint(QPainter * painter, const QStyleOptionGraphicsItem * option, QWidget * widget = 0);
}

class Group : public Base,QGraphicsItemGroup
{
public:
    Group(QGraphicsItem * parent = 0);
}

I can compile this code but when I add something like scene.addItem(item) when item is instance of Group or Item I get error:

request for member 'addItem' is ambiguous

And this error is understandable. There are 2 parent of type QGraphicsItem for the object and compiler just discouraged in this case. So my question - how can I avoid it? I don't want to copy same code (for scaling) into 2 classes.