А просто убрать
void doubleClicked ( const QModelIndex & index )
так как:
void QAbstractItemView::activated ( const QModelIndex & index ) [signal]
This signal is emitted when the item specified by index is activated by the user.
How to activate items depends on the platform; e.g., by single- or double-clicking the item,
or by pressing the Return or Enter key when the item is current.
See also clicked(), doubleClicked(), entered(), and pressed().