public TextBox Clone( bool cloneDescendants )
Public Function Clone ( cloneDescendants As Boolean ) As TextBox
Document content element instance can exist only in a one place in the document.
If you want to insert document content element into some other part of the same document, then clone the element and insert its clone.
If you want to insert document content element into another document, then you should first import it into another document with ImportT(T, Boolean, Boolean) method and then insert the imported element.
For more information, see cloning sample.