ListBox и ListBoxItems. Element is already the child of another element

public partial class MatrixBar : UserControl
    {
        public MatrixBar()
        {
            InitializeComponent();
 
            foreach (ListBoxItemMatrix lbim in lbimQueue)
                    lbMatrix.Items.Add(lbim);//здесь ошибка!
        }
 
//........
 
        public static Queue<ListBoxItemMatrix> lbimQueue = new Queue<ListBoxItemMatrix>();
    }
}

public partial class PageDet : PhoneApplicationPage
    {
        MatrixBar matrixBar;
 
        public PageDet()
        {
            InitializeComponent();
 
            matrixBar = new MatrixBar();
            LayoutRoot.Children.Add(matrixBar);
 
            System.Windows.Thickness location = new System.Windows.Thickness(0, -280, 0, 0);
            matrixBar.Margin = location;
            matrixBar.VerticalAlignment = System.Windows.VerticalAlignment.Top;
            matrixBar.HorizontalAlignment = System.Windows.HorizontalAlignment.Left;
 
            //.....
            matrixBar.lbMatrix.SelectionChanged += lbMatrix_SelectionChanged;
        }
//....
    }
}