xsd composite unique key

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Hi, I have an xml file like

<?xml version="1.0" encoding="utf-8" ?>
<Modules xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="CommonXSD.xsd">
  <Module name="SETUP">

  <Type Code="1" Description="Location"/>
    <Type Code="2" Description="Mode of Payment"/>

 </Module >
  <Module name="GMT">
    <Type Code="2" Description="Mode of Payment"/>
</Module>
</ Modules >

For this I have an xml schema like

< ?xml version="1.0" encoding="utf-8"? > 
< xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" attributeFormDefault="unqualified" elementFormDefault="qualified" > 
  < xs:element name="Modules" > 
    < xs:complexType > 
      < xs:sequence > 
        < xs:element name="Module" maxOccurs="unbounded" minOccurs="0" > 
          < xs:complexType mixed="true" > 
            < xs:sequence > 
              < xs:element name="Type" maxOccurs="unbounded" minOccurs="0" > 
                < xs:complexType > 
                  < xs:simpleContent > 
                    < xs:extension base="xs:string" > 
                      < xs:attribute type="xs:byte" name="Code" use="required"/ > 
                      < xs:attribute type="xs:string" name="Description" use="required"/ > 
                    < /xs:extension > 
                  < /xs:simpleContent > 
                < /xs:complexType > 
              < /xs:element > 
            < /xs:sequence > 
            < xs:attribute type="xs:string" name="name" use="required"/ > 
          < /xs:complexType > 
        < /xs:element > 
      < /xs:sequence > 
    < /xs:complexType > 

      
  < /xs:element > 
< /xs:schema > 

I want to create module wise type code unique key. i.e Each module may contain same type code but same module must not contain duplicate type code. Please help me thanks.