How many tags in one structure?

Hello,

assuming I have the following structure (please take my applogies if the
books don't suit your taste ;-):

<bookshelf>
   <book>
     <id>1</id>
     <title>Alice in Wonderland</title>
     <description>A classic</description>
   </book>
    <book>
     <id>2</id>
     <title>1984</title>
     <description>A British Classic about a completely controlled society
written by George Orwell.</description>
     <description>Very good read</description>
   </book>
</bookshelf>

I wouild like to get those books, which do have two "description" tags. How
can I do that? The results shouild look like that:

<bookshelf>
    <book>
     <id>2</id>
     <title>1984</title>
     <description>A British Classic ...</description>
     <description>Very good read</description>
   </book>
</bookshelf>

Kind Regards,
Karl

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