multiply-connected graph of schema import

Is a multiply-connected (but non-circular) graph of schema imports allowed by the standard?

e.g  B imports A
      C imports B
      D imports C and A


The intention is that schema D extends types that appear in schemas C and A. There seems to be some confusion in various tools as to what is needed - it seems that some tools require the explicit import of schema A if D makes an explicit extension of a type in schema A, but other tools will report that the types in A have been defined twice it this explicit import is made.

I would like a opinion on what should be happening according to the standards, so that I can complain to one of the schema tool writers that I am using.....

Regards,
	Paul