Hi Andrew,
I'll try to state the problem in words:

I need to create <FormatPages> from within the context of the
match="Formats" template. A <FormatPage> is created when the value of an
attribute within a <Formats> element matches the value of the
CatNumbers/@pofis-number. Call such a <Formats> element "the creator".
Having encountered such a match, I create the <FormatPage> and now want to
create its child <FormatButtons>. The children of <FormatButtons> are
selected as follows: For any <Stamp> in the set that contains "the creator",
get a copy of "the creator's" sibling <Formats> elements. My code does this.
Unfortunately, it does not distinguish between duplicates. This, for the
example I gave, it selects two copies of the <Formats se-tenant="365"/>
because I do not know how to filter the duplicate out. Before I ask anything
else, I am going to try to figure out how to use Michael's suggestion.