Identity Constraint Problem

Hi,
I have a schema and xml file below. There is a key constraint on the element 'person' that says the element 'person_id' should be a key. The XML document has 2 persons both with the same value for the element person_id - This should not be valid I believe.....

Can someone explain why this key constraint isn't working?

(I hope these get displayed since there's tags here)
Thanks,
Vmuisc

---------------------SCHEMA----------------------------
<?xml version="1.0" encoding="UTF-8"?>
<!-- edited with XMLSPY v5 rel. 4 U (http://www.xmlspy.com) by William Gattis (MSBS) -->
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified">
<xs:element name="persons">
<xs:complexType>
<xs:sequence>
<xs:element name="person" maxOccurs="unbounded">
<xs:complexType>
<xs:all>
<xs:element name="first_nm"/>
<xs:element name="last_nm"/>
<xs:element name="person_id" type="xs:integer"/>
</xs:all>
</xs:complexType>
<xs:key name="personKey">
<xs:selector xpath="."/>
<xs:field xpath="person_id"/>
</xs:key>
</xs:element>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>

-------------------- SAMPLE XML INSTANCE -----------------------------
<?xml version="1.0" encoding="UTF-8"?>
<persons xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="C:\a_work\junk_schemas\integrity constraints\persons.xsd">
<person>
<first_nm>Billy</first_nm>
<last_nm>Bob</last_nm>
<person_id>1</person_id>
</person>
<person>
<first_nm>Person</first_nm>
<last_nm>Two</last_nm>
<person_id>1</person_id>
</person>
</persons>

<!-- as you can see the value for person_id is the same and this should be invlaid , but I don't get any validation errros --->


<Thanks A Bunch/>