2 level grouping

I have a problem that I have been trying to find the solution for for 
quite a while, but frankly it's a bit too complicated for my limited 
xsl skills, maybe someone on this list can point me in the right 
direction.
I have something like this:

<items>
        <item month="5" year="2003"/>
        <item month="5" year="2003"/>
        <item month="4" year="2003"/>
        <item month="3" year="2003"/>
        <item month="2" year="2003"/>
        <item month="1" year="2003"/>
        <item month="12" year="2002"/>
        <item month="11" year="2002"/>
        <item month="10" year="2002"/>
        <item month="9" year="2002"/> 
</items>

And as a result I would like a list of all the months and all the 
years, with no duplicates. Something like this:
5/2003
4/2003
3/2003
2/2003
1/2003

5/2002

ect. you get the idea...

Now if it was just the months I could (with a little help from google) 
write an xsl that would work. But the years make it much more 
complicated.
Hopefully I'm making myself clear here...


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