when are duplicated type definitions allowed?

Hej,

I can't find any clear explanation in the schema 1.1 (1.0) spec how to handle
multiple imports of the same schema through different documents.

e.g. A imports B and C (with different namespaces)
       B imports D 
       C imports D
				(B and C import D with the same namespace)

so all the types/elements/attributes from D appear suddenly twice in the 
target schema... is that allowed?

Another situation: let's have the same type/element/attribute definitions
in two schema files with different filenames (URIs)
What should happen if a third schema includes (or imports) these two
files?

Thanks in advance

Ciao,
Tobias