Handling list of xsd:ID

If I have a xsd:list of xsd:ID values, how would the uniqueness check be
applied during validation? Consider the following:

<xsd:complexType name="base">
	 <xsd:attribute name="att1" type="myList"/>
</xsd:complexType>

<xsd:simpleType name="myList">
	<xsd:list itemType="xsd:ID"/>
</xsd:simpleType>

If I am validating an element 'doc' having this type. What should be the
outcome of validating the following instance?

<root>
	<doc att1="a b"/>
	<doc att1="b c"/>
</root>


Thanks,
Zafar