XML primary key constraint not working

I have declared user/url to be primary key. But my validator is still accepting the same value of url. What am I missing?

Is there some other way of specifying primary key constraint? or is my understanding of xsd insufficient?

Thanks

My xml:

<?xml version="1.0"?>

<user_list xmlns="http://www.cse532.com" 
xmlns:xsi="http://ift.tt/ra1lAU" 
xsi:schemaLocation="http://www.cse532.com schema.xsd">


<user>
    <name>Bob</name>
    <url>http://ift.tt/1uWZjGT;
    <work_history>
        <company>Stony Brook</company>
        <from_date>2013-08-01</from_date>
        <to_date>2013-12-31</to_date>
    </work_history>
    <work_history>
        <company>Microsoft</company>
        <from_date>2014-01-01</from_date>
        <to_date>2014-12-31</to_date>
    </work_history>
    <skills>
        <skill>Programming</skill>
    </skills>
    <endorsements>
        <url>http://ift.tt/1AC0Nv0;
        <skill>Programming</skill>
    </endorsements>
    <endorsements>
        <url>http://ift.tt/1AC0Nv0;
        <skill>Instructor</skill>
    </endorsements>
    <endorsements>
        <url>http://ift.tt/1uWZhia;
        <skill>Programming</skill>
    </endorsements>
</user>

<user>
    <name>Bob</name>
    <url>http://ift.tt/1uWZjGT;
    <work_history>
        <company>Stony Brook</company>
        <from_date>2013-08-01</from_date>
        <to_date>2013-12-31</to_date>
    </work_history>
    <work_history>
        <company>Microsoft</company>
        <from_date>2014-01-01</from_date>
        <to_date>2014-12-31</to_date>
    </work_history>
    <skills>
        <skill>Programming</skill>
    </skills>
    <endorsements>
        <url>http://ift.tt/1AC0Nv0;
        <skill>Programming</skill>
    </endorsements>
    <endorsements>
        <url>http://ift.tt/1AC0Nv0;
        <skill>Instructor</skill>
    </endorsements>
    <endorsements>
        <url>http://ift.tt/1uWZhia;
        <skill>Programming</skill>
    </endorsements>
</user>

my xsd:

 <!-- Definition of simple elements. -->
 <xs:element name="name" type="xs:string"/>
 <xs:element name="url" type="xs:string"/>
 <xs:element name="company" type="xs:string"/>
 <xs:element name="from_date" type="xs:string"/>
 <xs:element name="to_date" type="xs:string"/>
 <xs:element name="skill" type="xs:string"/>

<!-- Definition of complex elements. -->
<xs:element name="work_history">
    <xs:complexType>
        <xs:sequence>
            <xs:element ref="company"/>
            <xs:element ref="from_date"/>
            <xs:element ref="to_date"/>
        </xs:sequence>
    </xs:complexType>
</xs:element>

<xs:element name="skills">
    <xs:complexType>
        <xs:sequence>
            <xs:element ref="skill"/>
        </xs:sequence>
    </xs:complexType>
</xs:element>

<xs:element name="endorsements">
    <xs:complexType>
        <xs:sequence>
            <xs:element ref="url"/>
            <xs:element ref="skill"/>
        </xs:sequence>
    </xs:complexType>
</xs:element>

<xs:element name="user">
    <xs:complexType>
        <xs:sequence>
            <xs:element ref="name" maxOccurs="1"/>
            <xs:element ref="url"/>
            <xs:element ref="work_history" minOccurs="0" maxOccurs="unbounded"/>
            <xs:element ref="skills" minOccurs="0" maxOccurs="unbounded"/>
            <xs:element ref="endorsements" minOccurs="0" maxOccurs="unbounded"/>
        </xs:sequence>
    </xs:complexType>
</xs:element>

<!-- "user_list" is the root element of this schema. -->
<xs:element name="user_list">
    <xs:complexType>
        <xs:sequence>
            <xs:element ref="user" maxOccurs="unbounded"/>
        </xs:sequence>
    </xs:complexType>
    <!-- Primary key and foreign key definitions. -->
    <xs:key name="PKuser_url">
        <xs:selector xpath="user"/>
        <xs:field xpath="url"/>
    </xs:key>
    <xs:key name="PKwork_history">
        <xs:selector xpath="user/work_history"/>
        <xs:field xpath="company"/>
        <xs:field xpath="from_date"/>
    </xs:key>
    <xs:key name="PKskills">
        <xs:selector xpath="user/skills"/>
        <xs:field xpath="skill"/>
    </xs:key>
    <xs:key name="PKendorsements">
        <xs:selector xpath="user/endorsements"/>
        <xs:field xpath="url"/>
        <xs:field xpath="skill"/>
    </xs:key>
    <xs:keyref name="FKurl" refer="PKuser_url">
        <xs:selector xpath="user/endorsements"/>
        <xs:field xpath="url"/>
    </xs:keyref>
    <xs:keyref name="FKskill" refer="PKskills">
        <xs:selector xpath="user/endorsements"/>
        <xs:field xpath="skill"/>
    </xs:keyref>
</xs:element>
</xs:schema>