Unique Particle Attribution

>From my reading of the Unique Particle Attribution constraint in the
spec, the following schema is a violation:

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

<xsd:complexType name="type">     
<xsd:sequence>
	<xsd:element ref="a" minOccurs="0"/>
	<xsd:element ref="b" minOccurs="0"/>
	<xsd:element ref="a" maxOccurs="2"/>
</xsd:sequence>
</xsd:complexType>
<xsd:element name="a" />
<xsd:element name="b" />
</xsd:schema>


It can not be known which particle (a) to validate, even through they
are references to the same element schema component. Is this
understanding correct?

Thanks.