[xsl] Identifying unique attribute values in nested sibling elements

["Mark" <mark@xxxxxxxxxxxx>]

In processing XML similar to the fragment below, I have identified that 
within its <Set> element, five <Stamp> elements are of interest (because 
they all contain <Formats se-tenant="1">). The other possible <Stamp> 
elements that do not have this feature are of no interest. My problem occurs 
in <xsl:template match="Stamp">:

While I am in the context of the *unique* <Stamp> element that has identical 
values for the pofis-number and se-tenent attributes -- in this instance, 
the fragment's first <Stamp> element --  how can I gather all of the <Value> 
elements from the subsequent <Stamp> elements of interest while ignoring:
(1)  <Value> elements identical to those already found, and
(2) <Value> elements in <Stamp> elements of no interest?

For the given XML fragment, I want to find these four <Value> elements and 
ignore the duplicated <Value> element:
Unique:
kc-value="14"
kc-value="18"
kc-value="18" h-value="50"
kc-value="0" l-value="A"

Duplicate:
kc-value="14"

Thanks,
Mark
-----------------------------
<Set>
<!--possibly other unrelated <Stamp> elements here-->
<Stamp>
     <CatNumbers  pofis-number="1"/>
     <Value kc-value="14"/>
     <Formats se-tenant="1"/>
</Stamp>
<!--possibly other unrelated <Stamp> elements here-->
<Stamp>
     <CatNumbers pofis-number="2"/>
     <Value kc-value="18"/>
     <Formats se-tenant="1"/>
</Stamp>
..... <!--possibly other unrelated stamp elements-->
<Stamp>
     <CatNumbers pofis-number="3"/>
     <Value kc-value="18" h-value="50"/>
     <Formats se-tenant="1"/>
</Stamp>
<!--possibly other unrelated <Stamp> elements here-->
<Stamp>
     <CatNumbers pofis-number="4"/>
     <Value kc-value="14" />
     <Formats se-tenant="1"/>
</Stamp>
<!--possibly other unrelated <Stamp> elements here-->
<Stamp>
     <CatNumbers pofis-number="5"/>
     <Value kc-value="0" l-value="A" />
     <Formats se-tenant="1"/>
</Stamp>
</Set>