Unique Attribute value

Hello all,

Is there a way to check whether an element's attribute has unique value in 
xml structure and is not repeating. 

For instance, in below sample xml "p1" is repeating twice and any good 
parser will throw an error for it, though I would like it to check it with 
XSLT itself. Weird, I know. I am just trying to figure out the best 
possible way to get this done.

My actual requirement is little bit more complex but thought of starting 
asking with this. 


<?xml version="1.0"?>
<root>
        <x id="p1">xxx</x>
        <x id="p2">yyy</x>
    <b>
        <x id="p3">zzz</x>
        <x id="p1">AAA</x>
    </b>
</root>

TIA,
Pankaj