diff -Naur latex2html-master.orig/latex2html.pin latex2html-master/latex2html.pin
--- latex2html-master.orig/latex2html.pin	2021-06-05 18:58:52.279933335 +0700
+++ latex2html-master/latex2html.pin	2021-06-05 20:22:49.449880745 +0700
@@ -9852,6 +9852,7 @@
 sub make_name {
     local($sec_name, $packed_curr_sec_id) = @_;
     local($title,$making_name,$saved) = ('',1,'');
+    local($filename) = '';
     if ($LONG_TITLES) {
 	$saved = $_;
 	&process_command($sections_rx, $_) if /^$sections_rx/;
@@ -9869,7 +9870,16 @@
 	#ensure no more than 32 characters, including .html extension
 	$title =~ s/^(.{1,27}).*$/$1/;
     	++$OUT_NODE;
-	join("", ${PREFIX}, $title, $EXTN);
+	$filename = join("", ${PREFIX}, $title, $EXTN);
+	# Avoid duplication of filenames
+	unless ($nodenames{$filename}) {
+	    $nodenames{$filename} = 1;
+	} else {
+	    # Duplication detected: add version number to generated filename
+	    $filename =
+		join("", ${PREFIX}, $title, ++$nodenames{$filename}, $EXTN);
+	}
+	$filename;
     } else {
     # Remove 0's from the end of $packed_curr_sec_id
 	$packed_curr_sec_id =~ s/(_0)*$//;
diff -Naur latex2html-master.orig/tests/longtitles.tex latex2html-master/tests/longtitles.tex
--- latex2html-master.orig/tests/longtitles.tex	1970-01-01 07:00:00.000000000 +0700
+++ latex2html-master/tests/longtitles.tex	2021-06-05 20:16:59.129884402 +0700
@@ -0,0 +1,66 @@
+\documentclass[10pt]{article}
+\usepackage[T1]{fontenc}
+\usepackage[utf8]{inputenc}
+\usepackage[english]{babel}
+
+% Testing longtitles option
+%
+% pdflatex longtitles (may be do it several times to be sure)
+%
+% latex2html -html_version 4.0 -local_icons longtitles.tex
+% Everything should be OK. There are several sections with duplicated names.
+% The duplications are intentional, to demonstrate the bug.
+% Notice section 1, subsections 1.1 and 1.2,
+% section 2, subsection 2.1, and section 3, subsection 3.1.
+%
+% rm -rf longtitles
+% latex2html -html_version 4.0 -local_icons -long_titles 5 longtitles.tex
+% Now you will not see sections 1 and 2 at all, and section 3 will be mess.
+% Moreover, repetitive clicking 'Next' in the browser will never reach the end.
+% This is because of mutual clobbering of duplicated HTML filenames.
+%
+% After applying the patch against titles duplication
+% the -long_titles option should work correctly:
+% the sequential numbers will be added to the duplicated filenames
+% starting from #2.
+
+\begin{document}
+
+\tableofcontents
+
+\section{Some basic stuff with very long first section name}
+
+This is some simple text, from section \thesection{} Some basic stuff
+with very long first section name.
+
+\subsection{Fonts}
+
+This subsections has some fonts, from section \thesection{},
+subsection \thesubsection{} Fonts.
+
+\subsection{Lists}
+
+This subsections has some lists, from section \thesection{},
+subsection \thesubsection{} Lists.
+
+\section{Some basic stuff with very long second section name}
+
+This is some simple text, from section \thesection{} Some basic stuff
+with very long second section name.
+
+\subsection{Fonts}
+
+This subsections has some fonts, from section \thesection{},
+subsection \thesubsection{} Fonts.
+
+\section{Some basic stuff with very long third section name}
+
+This is some simple text, from section \thesection{} Some basic stuff
+with very long third section name.
+
+\subsection{Lists}
+
+This subsections has some lists, from section \thesection{},
+subsection \thesubsection{} Lists.
+
+\end{document}