Follwing is a "pure regex" approach. It may of interest/useful if:

• All text will fit in available memory (a line-by-line approach has no inherent memory limit);
• eyepopslikeamosquito's wise caution (really haukex's) against using regex to parse (X|HT)ML is heeded;
• Speed is not critical (I suspect the regex engine will be heavily loaded and line-by-line will be quicker, but only Benchmark-ing will tell the tale).

Win8 Strawberry 5.8.9.5 (32)  Fri 08/11/2023 14:59:13
C:\@Work\Perl\monks
>perl

use strict;
use warnings;

use Test::More;
use Test::NoWarnings;

my @Tests = (

'ALL these should match',

#                       match
#  string               success
[ '<FILENAME>.htm' =>     1, ],

[ '<FILENAME>dp198076_424b2-us2342673.htm' => 1, ],

[ 'fizz
<FILENAME>dp198076_424b2-us2342673.htm
fuzz' => 1, ],

'NONE of these should match',

[ '' => '', ],

[ ' ' => '', ],

[ 'xxx' => '', ],

[ "\n\n\n" => '', ],

[ '<FILENAME>dp198076_424b2-us2342673.htm
<FILENAME>dp198076_exfilingfees.htm' => '', ],

[ '<FILENAME>.htmfred' => '', ],  # see pm#11153807

[ 'fizz
<FILENAME>dp198076_424b2-us2342673.htm
foo bar
<FILENAME>dp198076_exfilingfees.htm
fuzz' => '', ],

[ 'fizz
<FILENAME>dp198076_424b2-us2342673.htm
foo <FILENAME>xxx.htm bar
<FILENAME>dp198076_exfilingfees.htm
fuzz' => '', ],

[ '<FILENAME>xxx.htm<FILENAME>yyy.htm' => '', ],

[ '<FILENAME>.htm<FILENAME>.htm' => '', ],

  );  # end @Tests

my @additional = qw(Test::NoWarnings);  # each of these adds 1 test
plan 'tests' => (scalar grep { ref eq 'ARRAY' } @Tests)
              + @additional
              ;

my $rx_once = qr{ <FILENAME> (?: (?! [.]htm) .)* [.]htm \b }xms;

VECTOR:
for my $ar_vector (@Tests) {

    if (not ref $ar_vector) {
        note $ar_vector;
        next VECTOR;
        }

    my ($string, $expected) = @$ar_vector;

    my $got = $string =~ m{
        \A
        (?: (?! $rx_once) .)*  # no match before single match
        $rx_once               # match just once
        (?: (?! $rx_once) .)*  # no match after single match
        \Z
        }xms;

    is $got, $expected;

    }  # end for VECTOR

^Z
1..14
# ALL these should match
ok 1
ok 2
ok 3
# NONE of these should match
ok 4
ok 5
ok 6
ok 7
ok 8
ok 9
ok 10
ok 11
ok 12
ok 13
ok 14 - no warnings
[download]

Update: Note that as the code stands, the test vector
    [ '<FILENAME>.htm<FILENAME>.htmfred' => 1, ]
will match/pass, i.e., will be accepted as having no duplication. Should this be the case?