Specified element is already a logical child of another element - wpf

Question

I hope this doesn't get marked as duplicate since my problem is kinda complex, so none of the other answers helped. I have a class called 'ControlChoiceModule' that generates a System.Windows.Controls object based on the type of property it deals with (String - TextBox, Boolean - CheckBox, DateTime - DatePicker, etc..).
It has two dictionaries:
public static class ControlChoiceModule
{
private static readonly Dictionary<Type, object> TypeToControl = new Dictionary<Type, object>
{
{typeof(bool), new CheckBox() },
{typeof(DateTime), new DatePicker() }
};
private static readonly Dictionary<Type, DependencyProperty> ControlToProperty = new Dictionary<Type, DependencyProperty>
{
{typeof(TextBox), TextBox.TextProperty },
{typeof(CheckBox), CheckBox.IsCheckedProperty },
{typeof(DatePicker), DatePicker.SelectedDateProperty }
};
The purpose of the other one is just for binding. And here are the two methods:
public static object GenerateControl(Type theType, Binding B)
{
object O;
if (TypeToControl.ContainsKey(theType))
{
O = TypeToControl[theType];
}
else
{
O = new TextBox();
}
SetBinding(O, B);
return O;
}
private static void SetBinding(object O, Binding B)
{
BindingOperations.SetBinding(O as DependencyObject, ControlToProperty[O.GetType()], B);
}
Now the purpose of all this is to generate an insertion window for a certain class, generically. So the windows loops through all of the class's properties and, based on the type, generates an appropriate field for it.
private void GenerateInsertionOrUpdatePage(string windowText)
{
var w2 = new InsertionWindow();
w2.DataContext = this.DataContext;
w2.Title = windowText;
w2.Show();
foreach (var P in ReturnPropertyList())
{
if (P.Name != "SearchableString" && P.Name != "Id" )
{
Label L = new Label();
L.Content = P.Name + ":";
w2.InsertionStackPanel.Children.Add(L);
Binding B = new Binding();
B.Path = new PropertyPath("NewT." + P.Name);
B.UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged;
B.Mode = BindingMode.TwoWay;
**w2.InsertionStackPanel.Children.Add(ControlChoiceModule.GenerateControl(P.PropertyType, B) as UIElement);**
}
}
}
The method above gets called on a click of a button. The first time I click it, it works just fine. But when I click it again, I get the error from the title (on the line marked with **).
Any idea why this happens?
Thanks