Select elements from XML using Xpath

I have a huge XML file. Looks somthing like the following

<component name='comp1'>
  <child> ... </child>
  <child> ... </child>
  <optional id='1'> ... </optional>
</component>

<component name='comp2'>
  <child> ... </child>
  <child> ... </child>
  <optional id='1'> ... </optional>  
  <optional id='1'> ... </optional>
  <optional id='2'> ... </optional>
</component>

I want to select optional elements based on their ids from each component without repeating any id within the same component.

Simply, I want the following output for comp2, for example:

Comp2
1
2

My problem is to select optional elements that has similar id one time only.

I am using Xpath with JET1 (java Emitter Template) I am iterating each component looking for optional elements and printing their text(). However, my loop is resulting in the following

 Comp2
    1
    1
    2

I want to eliminate duplicated results.

My JET temple:

<c:iterate select="//component/optional" var="Optional">
<c:get select="$Optional"/>
</c:iterate>