Debugging Silverlight “element is already the child of another element” !

The problem I know how many people are having a problem with the catch-all XAML error “Element is already the child of another element” – it’s been a popular post on this blog and I come across it all of the time.  The real problem is that the exception gives no indication of the area causing the failure, you are forced into playing with the source file and banging your head against the wall.