"Element is already the child of another element" Why am I getting this error message?

public void PaintButtons()
        {
            IList p1numlist = GenerateRandom();
            IList p2numlist = GenerateRandom(); 
            p1b1.Content = images.ElementAt((int)p1numlist[0]);
            p1b2.Content = images.ElementAt((int)p1numlist[1]); 
            p1b3.Content = images.ElementAt((int)p1numlist[2]);
            p2b1.Content = images.ElementAt((int)p2numlist[0]); //This is the line where the exception is generated. Exception: System.InvalidOperationException, Message: Element is already the child of another element
            p2b2.Content = images.ElementAt((int)p2numlist[1]);
            p2b3.Content = images.ElementAt((int)p2numlist[2]);
            //p1b1,p1b2,p1b3,p2b1,p2b2,p2b3 are buttons
            //GenerateRandom() method returns a List of unique random numbers of size 3
            
        }

// I am adding images to the <code>images</code> collection using the following code
 
        IList<Image> images = new List<Image>();
        
        public Page1()
        {
            InitializeComponent();
            LoadImages();
        }
        public void LoadImages()
        {
            Uri[] uri = new Uri[3];
            uri[0] = new Uri("/BtnPics/Blue.png", UriKind.Relative);
            uri[1] = new Uri("/BtnPics/Red.png", UriKind.Relative);
            uri[2] = new Uri("/BtnPics/Yellow.png", UriKind.Relative);
            foreach (Uri u in uri)
            {
                BitmapImage bmp = new BitmapImage(u);
                images.Add(new Image() { Source = bmp });
                
            }
        }

<grid height="800" name="LayoutRoot" width="478">
        <grid.columndefinitions>
            <columndefinition width="65*" />
            <columndefinition width="413*" />
        </grid.columndefinitions>
        <Button Height="122" HorizontalAlignment="Left" Margin="-12,682,0,0" Name="p1b1" VerticalAlignment="Top" Width="170" Grid.ColumnSpan="2" TabIndex="3" />
        <Button Height="122" HorizontalAlignment="Left" Margin="85,682,0,0" Name="p1b2" VerticalAlignment="Top" Width="170" Grid.Column="1" TabIndex="2" />
        <Button Height="122" HorizontalAlignment="Left" Margin="248,682,0,0" Name="p1b3" VerticalAlignment="Top" Width="170" Grid.Column="1" TabIndex="1" />
        <textblock height="68" horizontalalignment="Left" margin="60,618,0,0" name="player1textBlock" text="TextBlock" verticalalignment="Top" width="223" grid.column="1" />
        <textblock height="68" horizontalalignment="Left" margin="61,116,0,0" name="player2textBlock" text="TextBlock" verticalalignment="Top" width="223" grid.column="1" />
        <Image Height="220" HorizontalAlignment="Left" Margin="45,260,0,0" Name="image1" Stretch="Fill" VerticalAlignment="Top" Width="250" Grid.Column="1" />
        <Button Grid.ColumnSpan="2" Height="122" HorizontalAlignment="Left" Margin="-12,-4,0,0" Name="p2b3" TabIndex="3" VerticalAlignment="Top" Width="170" />
        <Button Grid.Column="1" Height="122" HorizontalAlignment="Right" Margin="0,-4,158,0" Name="p2b2" TabIndex="2" VerticalAlignment="Top" Width="170" />
        <Button Grid.Column="1" Height="122" HorizontalAlignment="Left" Margin="248,-4,0,0" Name="p2b1" TabIndex="1" VerticalAlignment="Top" Width="170" />
    </grid>