default-duplicate-content-type
If present, this element indicates that its parent
content-type element represents the default
duplicate content type of this publication and may be treated
accordingly in client applications. This element only has meaning as
the child of a content-type element, and it
should not appear more than once in a
content-type resource. The result of
specifying the element more than once is undefined.
A non-binary content item cannot be "duplicated" into a binary content item.
Syntax
<default-duplicate-content-type/>