XSD atribute repeated

<?xml version="1.0" encoding="windows-1252"?>
<file xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://bibtexml.sf.net/ esquema_mas_completo.xsd"
 xmlns="http://bibtexml.sf.net/">

  <entry id="abc">
    <article>
      <author>Menindez, Isabel</author>
      <title>Actuar con prudencia</title>
     </article>
  </entry>

  <entry id="abc">
    <book>
      <author>Boyne, John</author>
      <year>2006</year>
     </book>
  </entry>

  <xs:element name="entry">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="ns1:article" minOccurs="0" maxOccurs="unbounded"/>
        <xs:element ref="ns1:book" minOccurs="0" maxOccurs="unbounded"/>
      </xs:sequence>
      <xs:attribute name="id" use="required" type=" xs:token"/>
    </xs:complexType>
  </xs:element>

 I need that the XSD file not validate the XML file if in the XML
appears for example two atributes that have the same value. In the
example of xml, the atribute id is the same in both of entry (abc).