How to display duplicated attribute value using xslt

I am a newbie in xslt and i want to ask for a help in creating an .xsl file that will validate if the attribute value has the same value.

<catalog>
    <book id="bk101">aa</book>
    <book id="bk102">bb</book>
    <book id="bk103">cc</book>
    <book id="bk101">dd</book>
</catalog>

choose when the value of the '@id' is not same as the other '@id'value then it will display: aa bb cc dd other wise it will display:

your '@id' has same value of 'bk101'