Hi Kevin:

here is an example that has duplicate parts:

<xsd:complexType name="C">
  <xsd:complexContent>
    <xsd:extension base="E">
      <xsd:sequence>
        <xsd:element name="op" type="Op"/>
        <xsd:element name="arg" type="Exp"/>
        <xsd:element name="arg" type="Exp"/>
      </xsd:sequence>
    </xsd:extension>
  </xsd:complexContent>
</xsd:complexType>

In class dictionary notation we would write:

C = <op> Op
    <arg> Exp
    <arg> Exp.
E : C.

In other words, you have to check whether all names in
a sequence are unique. This is the translation of the "unique parts"
check for class dictionaries.

Please note that

<xsd:complexType name="A">
  <xsd:sequence>
      <xsd:element name="b" type="B"/>
  </xsd:sequence>
</xsd:complexType>

<xsd:complexType name="B">
	  <xsd:sequence>
	      <xsd:element name="b" type="C" />
          </xsd:sequence>
</xsd:complexType> 

(or

A = <b> B.
B = <b> C.
)

DOES NOT violate the unique parts rule.