diff --git a/administrator/components/com_admin/postinstall/htaccessbrotli.php b/administrator/components/com_admin/postinstall/htaccessbrotli.php
index 7063b0f6906a6..58f7bb02b37b9 100644
--- a/administrator/components/com_admin/postinstall/htaccessbrotli.php
+++ b/administrator/components/com_admin/postinstall/htaccessbrotli.php
@@ -26,5 +26,11 @@
  */
 function admin_postinstall_htaccessbrotli_condition()
 {
-    return true;
+    $htaccessContent = '';
+
+    if (is_file(JPATH_ROOT . '/.htaccess') || is_file(JPATH_ROOT . '/htaccess.txt')) {
+        $htaccessContent = file_get_contents(is_file(JPATH_ROOT . '/.htaccess') ? JPATH_ROOT . '/.htaccess' : JPATH_ROOT . '/htaccess.txt');
+    }
+
+    return !str_contains($htaccessContent, 'E=no-brotli:1');
 }
diff --git a/administrator/components/com_admin/sql/updates/mysql/5.1.0-2024-03-28.sql b/administrator/components/com_admin/sql/updates/mysql/5.1.0-2024-03-28.sql
index 2c3f6ec6e7884..e76716010f9fd 100644
--- a/administrator/components/com_admin/sql/updates/mysql/5.1.0-2024-03-28.sql
+++ b/administrator/components/com_admin/sql/updates/mysql/5.1.0-2024-03-28.sql
@@ -1,5 +1,12 @@
 --
 -- Add post-installation message about Brotli compression in .htaccess
 --
-INSERT IGNORE INTO `#__postinstall_messages` (`extension_id`, `title_key`, `description_key`, `action_key`, `language_extension`, `language_client_id`, `type`, `action_file`, `action`, `condition_file`, `condition_method`, `version_introduced`, `enabled`)
-SELECT `extension_id`, 'COM_ADMIN_POSTINSTALL_MSG_HTACCESS_BROTLI_TITLE', 'COM_ADMIN_POSTINSTALL_MSG_HTACCESS_BROTLI_DESCRIPTION', '', 'com_admin', 1, 'message', '', '', 'admin://components/com_admin/postinstall/htaccessbrotli.php', 'admin_postinstall_htaccessbrotli_condition', '5.1.0', 1 FROM `#__extensions` WHERE `name` = 'files_joomla';
+-- This statement had to be modified to prevent duplicate postinstall messages
+-- when updating from 4.4.
+-- See https://github.com/joomla/joomla-cms/pull/43182 for details.
+--
+INSERT INTO `#__postinstall_messages` (`extension_id`, `title_key`, `description_key`, `action_key`, `language_extension`, `language_client_id`, `type`, `action_file`, `action`, `condition_file`, `condition_method`, `version_introduced`, `enabled`)
+SELECT `extension_id`, 'COM_ADMIN_POSTINSTALL_MSG_HTACCESS_BROTLI_TITLE', 'COM_ADMIN_POSTINSTALL_MSG_HTACCESS_BROTLI_DESCRIPTION', '', 'com_admin', 1, 'message', '', '', 'admin://components/com_admin/postinstall/htaccessbrotli.php', 'admin_postinstall_htaccessbrotli_condition', '5.1.0', 1
+  FROM `#__extensions`
+ WHERE `name` = 'files_joomla'
+   AND (SELECT COUNT(a.`postinstall_message_id`) FROM `#__postinstall_messages` a WHERE a.`title_key` = 'COM_ADMIN_POSTINSTALL_MSG_HTACCESS_BROTLI_TITLE') = 0;
diff --git a/administrator/components/com_admin/sql/updates/postgresql/5.1.0-2024-03-28.sql b/administrator/components/com_admin/sql/updates/postgresql/5.1.0-2024-03-28.sql
index 6858e2b668b28..82bf0d612608c 100644
--- a/administrator/components/com_admin/sql/updates/postgresql/5.1.0-2024-03-28.sql
+++ b/administrator/components/com_admin/sql/updates/postgresql/5.1.0-2024-03-28.sql
@@ -1,6 +1,12 @@
 --
 -- Add post-installation message about Brotli compression in .htaccess
 --
+-- This statement had to be modified to prevent duplicate postinstall messages
+-- when updating from 4.4.
+-- See https://github.com/joomla/joomla-cms/pull/43182 for details.
+--
 INSERT INTO "#__postinstall_messages" ("extension_id", "title_key", "description_key", "action_key", "language_extension", "language_client_id", "type", "action_file", "action", "condition_file", "condition_method", "version_introduced", "enabled")
-SELECT "extension_id", 'COM_ADMIN_POSTINSTALL_MSG_HTACCESS_BROTLI_TITLE', 'COM_ADMIN_POSTINSTALL_MSG_HTACCESS_BROTLI_DESCRIPTION', '', 'com_admin', 1, 'message', '', '', 'admin://components/com_admin/postinstall/htaccessbrotli.php', 'admin_postinstall_htaccessbrotli_condition', '5.1.0', 1 FROM "#__extensions" WHERE "name" = 'files_joomla'
-ON CONFLICT DO NOTHING;
+SELECT "extension_id", 'COM_ADMIN_POSTINSTALL_MSG_HTACCESS_BROTLI_TITLE', 'COM_ADMIN_POSTINSTALL_MSG_HTACCESS_BROTLI_DESCRIPTION', '', 'com_admin', 1, 'message', '', '', 'admin://components/com_admin/postinstall/htaccessbrotli.php', 'admin_postinstall_htaccessbrotli_condition', '5.1.0', 1
+  FROM "#__extensions"
+ WHERE "name" = 'files_joomla'
+   AND (SELECT COUNT(a."postinstall_message_id") FROM "#__postinstall_messages" a WHERE a."title_key" = 'COM_ADMIN_POSTINSTALL_MSG_HTACCESS_BROTLI_TITLE') = 0;