How does WSDL import work?

I've been going through the WSDL spec trying to figure out <wsdl:import>
and I can't get a clear reading on it:

1.  I understand that message/types/portType sections can exist in
external files and can be imported into the main wsdl.  Is it legal to
import a given message/types/portType section more than once from
multiple documents?  In other words, can some of my <message>
definitions come from one document and others from another?

2.  Anybody know of a clear description of wsdl:import semantics?

Regards,
Jim

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