First occurence of a repeating value

<main>
 <sub>
    <entry>
       <item1>val1</item1>
       <item2>val2</item2>
       <item3>val3</item3>
    </entry>
    <entry>
       <item1>val4</item1>
       <item2>val2</item2>
       <item3>val5</item3>
    </entry>
    <entry>
       <item1>val6</item1>
       <item2>val2</item2>
       <item3>val7</item3>
    </entry>
  </sub>
</main>

I want the output to be as shown below, using "val2" once as the title,
and listing all the other values after that:

Title: val2
------
Item1: val1
Item3: val3

Item4: val4
Item5: val5

Item6: val6
Item7: val7