[xml] including and importing schemas

[Luigi Selmi <l selmi ancitel it>]

Hi All,
we are using libxml2 in order to validate our xml schemas. The xsd files 
are organized as stated in the following:

  * RichiestaSAIA.xsd (targetNamespacePrefix: ap5) includes 
ComAnagTypes.xsd and imports RichiestaAPR4.xsd
  * ComAnagTypes (targetNamespacePrefix: ap5) includes 
RichiestaMacroTypes.xsd
  * RichiestaMacroTypes.xsd (targetNamespacePrefix: ap5) includes 
BaseTypes.xsd
  * RichiestaAPR4.xsd (targetNamespacePrefix: apr4) imports BaseTypes.xsd

A picture of the schemas structure has been attached. The libxml2 
validation function reports the following error:

/Warning: DOMDocument::schemaValidate(): Element 
'{http://www.w3.org/2001/XMLSchema}import': The schema document 
'BaseTypes.xsd' cannot be imported, since it was already included or 
redefined. /

All the other validation tools, W3C Schema validation service, XMLSpy, 
XML-Beans, validate the schemas without reporting any error or warning. 
Can anyone suggest an explanation for this problem ? Thanks in advance.

Luigi