Multiple include problem in XML-Schema (XSD).

Hi,
Is there a mechanism in XML-Schema specification by which I can stop
multiple includes of a Schema (XSD)?
I have the following scenario.
Schema-A.xsd "includes" "CustomDatatypes.xsd"
Schema-B.xsd "includes" "CustomDatatypes.xsd"
Schema-C.xsd "includes" "CustomDatatypes.xsd"
"Schema-D.xsd" "includes" "Schema-A.xsd" AND "Schema-B.xsd" AND
"Schema-C.xsd".
This causes "redefinition" errors for the datatypes in the
"CustomDatatypes.xsd" because it is included multiple times. Is there a
syntax in XMLSchema language to work around this. Unfortunately I need to
have the schemas:Schema-A, Schema-B, Schema-C in separate documents because
I have to validate instances of documents that confirm to these schemas
alone.
Can any one suggest a solution or any thing I am doring wrong or missing.
Best Regards,
Venkat