For anyone keeping score, here is the final subroutune:

# As each tweak tag is processed, this subroutine is called.  Here we 
+will
# see if any previous tweak tag had the same name attribute.  If so, w
+e will
# replace the previous tweak tag with this new tweak tag's contents.
sub pruner {
    my ($twig, $this_tweak) = @_;
    my $tweak_name = $this_tweak->att('name');
    my $previous_tweak = $this_tweak->prev_sibling( qq{tweak[\@name="$
+tweak_name"]});

    # If the tweak's name is found elsewhere, replace the first
    # instance with the latest one, then delete the latest one.
    if ($previous_tweak) {
        print "\tReplacing\n\t\t" if $debug;
        $previous_tweak->print if $debug;
        print "\n\twith\n\t\t" if $debug;
        $this_tweak->print if $debug;
        print "\n" if $debug;
        $this_tweak->cut;
        $this_tweak->replace($previous_tweak);
    }
}
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