UPA violation or not?

Xerces reports this schema as a UPA violation, XSV reckons it is OK. Which
is correct?

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
	<xs:complexType name="bar">
		<xs:sequence>
		  <xs:element ref="pongo" minOccurs="0" maxOccurs="2"/>
		  <xs:element ref="pongo" minOccurs="0" maxOccurs="2"/>
		</xs:sequence>
  	</xs:complexType>
	<xs:element name="doc" type="bar"/>
	<xs:element name="pongo" type="xs:string"/>
</xs:schema>

Saxon 8.1 reports it as ambiguous, my current build of Saxon 8.2 doesn't (an
unintentional side-effect of an unrelated change). 

My reading of the spec is that it probably is ambiguous, but that it really
doesn't need to be.

Michael Kay