xsl:if or template?

I'm having trouble with the specifics on using XSL to format my data
properly.  The problem is that I need to display groups of like data.   I
have written functions that format the dates and times for me, but I am
having trouble only printing the like data once. For example, I am sorting
by the <Tracker_ID> and then the <Event_GMT>.  I want to then display the
Tracker_ID if it is unique followed by all the data for the specific
tracker.  I have been working with scripts, xsl:if and templates to try and
solve this. Any help would be greatly appreciated.
BoJo Allen

Sample data:
<EventData xmlns:dt="urn:schemas-microsoft-com:datatypes">
   <Event>
       <Event_GMT dt:dt="dateTime">2000-03-14T02:15:21</Event_GMT>
       <Description>Bob</Description>
       <Engineering>
                <Tracker_ID>1477</Tracker_ID>
        </Engineering>
   </Event>
   <Event>
       <Event_GMT dt:dt="dateTime">2000-03-14T23:12:32</Event_GMT>
       <Description>Joe</Description>
       <Engineering>
                <Tracker_ID>1477</Tracker_ID>
        </Engineering>
   </Event>
   <Event>
       <Event_GMT dt:dt="dateTime">2000-03-15T09:57:21</Event_GMT>
       <Description>Sam</Description>
       <Engineering>
                <Tracker_ID>1477</Tracker_ID>
        </Engineering>
   </Event>
   <Event>
       <Event_GMT dt:dt="dateTime">2000-03-14T10:11:47</Event_GMT>
       <Description>George</Description>
       <Engineering>
                <Tracker_ID>2255</Tracker_ID>
        </Engineering>
   </Event>
</EventData>


Desired output:
1477
       03-14-2000
                  2:15:21 AM - Bob
                  11:12:32 PM - Joe
      03-25-2000
                  9:57:21 AM - Sam
2255
       03-14-2000
                  10:11:47 AM - George


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