A curious property about dependency properties

For my Panzer General game, I have a collection of hexes that make up the main board. Before every turn, I update the main canvas with this collection. I wrote some code like this:

            this.MainCanvas.Children.Clear();
            foreach (Hex hex in Game.CurrentBoard)
            {
                hex.HexInfoTextBlock.Text = string.Empty;
                this.MainCanvas.Children.Add(hex);
            }

However, when I run it, I get the following exception:

System.InvalidOperationException was unhandled

Message=Element is already the child of another element.

My first thought was to create a Clone for each Hex – remove the old Hex and then add the new one to the collection. However, I still got the same error.

After mucking around Bing for a bit (and not getting any closer to the answer), I decided to back into the parent and try and clear the collection that way:

            Canvas currentCanvas = Game.CurrentBoard[0].Parent as Canvas;
            if (currentCanvas != null)
            {
                currentCanvas.Children.Clear();
            }

Surprise! Surprise! It worked. I am guessing that the dependency property of the children still hold onto their parent even after the clear until the page goes out of scope.