Multiple and circular import/include

Hi,

I have a question on multiple and circular import/include each, if
somebody could help.

Is it an error to import/include a schema more than once?
A import/includes B1 and B2.
Both B1, and B2, import/includes C.
What is the expected behaviour of the processor in such a case?

Are circular import/includes allowed?
A import/includes B, and B import/includes A?
What is the expected behaviour of the processor in such a case?

Thanks,
Vivek.