If you do not want to allow two Report elements with the same Region attribute value in general then you can define a unique constraint e.g.
Code:
<xs:element name="Regional">
<xs:complexType>
<xs:sequence maxOccurs="unbounded">
<xs:element name="Report">
...
</xs:element>
</xs:sequence>
</xs:complexType>
<xs:unique name="unique-region">
<xs:selector xpath="Report"/>
<xs:field xpath="@Region"/>
</xs:unique>
</xs:complexType>