Outputing Differences between two files

I'm trying to write an XSL that receives an XML file
as input and outputs only the values that have a
certain sibling's value duplicated in the file.  For
example, if I have the following XML:

<a>
   <b>Value1</b>
   <bb>Value2</bb>
   <bbb>Value3</bbb>
</a>
<c>
   <b>Value4</b>
   <bb>Value2</bb>
   <bbb>Value5</bbb>
</c>

In this example, I want the output sibling to be
"a/bbb".  Since "Value2" is shown twice for element
bb, I want to output "Value3", since that is the
output sibling.  Any feedback regarding this example
would be appreciated, so that I can apply it to my
actual situation.

Thanks.

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