only display if subnodes occur more than once

Hi,

I'm stuck again with my XSL. My XML Structure looks like:

<root>
	<sub_a>
		<elem_1/>
		<elem_2/>
		<elem_3/>
	</sub_a>
	<sub_b>
		<elem_1/>
		<elem_2/>
		<elem_2/>
		<elem_2/>
		<elem_3/>
	</sub_b>
	<sub_c>
		<elem_1/>
		<elem_2/>
		<elem_3/>
	</sub_c>
</root>

The thing I want to do is display the element sub_b, because it has
subnodes which occur more than once (elem_2). I really have no idea how
to test for this - playing around for nearly two hours now. Im using a
loop like <xsl:for-each select="/root/*"> and the output should look
like

sub_b (this is the header)
elem_2
elem_2 (these are the 3 values of elem_2)
elem_2

Best regards,
Clemens Prerovsky