diff --git a/includes/message_parser.php b/includes/message_parser.php
index 74b8e764..07774c86 100644
--- a/includes/message_parser.php
+++ b/includes/message_parser.php
@@ -1849,83 +1849,59 @@ class parse_message extends bbcode_firstpass
 		$attachment_data = $request->variable('attachment_data', array(0 => array('' => '')), true, \phpbb\request\request_interface::POST);
 		$this->attachment_data = array();
 
-		$check_user_id = ($check_user_id === false) ? $user->data['user_id'] : $check_user_id;
-
 		if (!count($attachment_data))
 		{
-			return;
+			return true;
 		}
 
-		$not_orphan = $orphan = array();
+		$positions = array();
 
 		foreach ($attachment_data as $pos => $var_ary)
 		{
-			if ($var_ary['is_orphan'])
-			{
-				$orphan[(int) $var_ary['attach_id']] = $pos;
-			}
-			else
-			{
-				$not_orphan[(int) $var_ary['attach_id']] = $pos;
-			}
-		}
-
-		// Regenerate already posted attachments
-		if (count($not_orphan))
-		{
-			// Get the attachment data, based on the poster id...
-			$sql = 'SELECT attach_id, is_orphan, real_filename, attach_comment, filesize
-				FROM ' . ATTACHMENTS_TABLE . '
-				WHERE ' . $db->sql_in_set('attach_id', array_keys($not_orphan)) . '
-					AND poster_id = ' . $check_user_id;
-			$result = $db->sql_query($sql);
-
-			while ($row = $db->sql_fetchrow($result))
-			{
-				$pos = $not_orphan[$row['attach_id']];
-				$this->attachment_data[$pos] = $row;
-				$this->attachment_data[$pos]['attach_comment'] = $attachment_data[$pos]['attach_comment'];
-
-				unset($not_orphan[$row['attach_id']]);
-			}
-			$db->sql_freeresult($result);
-		}
-
-		if (count($not_orphan))
-		{
-			trigger_error('NO_ACCESS_ATTACHMENT', E_USER_ERROR);
+			$positions[(int) $var_ary['attach_id']] = $pos;
 		}
 
-		// Regenerate newly uploaded attachments
-		if (count($orphan))
-		{
-			$sql = 'SELECT attach_id, is_orphan, real_filename, attach_comment, filesize
-				FROM ' . ATTACHMENTS_TABLE . '
-				WHERE ' . $db->sql_in_set('attach_id', array_keys($orphan)) . '
-					AND poster_id = ' . $user->data['user_id'] . '
-					AND is_orphan = 1';
-			$result = $db->sql_query($sql);
+		$check_user_id = ($check_user_id === false) ? $user->data['user_id'] : $check_user_id;
 
-			while ($row = $db->sql_fetchrow($result))
+		// Get the attachment data, based on the poster id...
+		$sql = 'SELECT attach_id, is_orphan, real_filename, attach_comment, filesize
+			FROM ' . ATTACHMENTS_TABLE . '
+			WHERE ' . $db->sql_in_set('attach_id', array_keys($positions)) . '
+				AND (
+					(poster_id = ' . $check_user_id . ' AND is_orphan = 0)
+					OR
+					(poster_id = ' . $user->data['user_id'] . ' AND is_orphan = 1)
+				)';
+		$result = $db->sql_query($sql);
+
+		while ($row = $db->sql_fetchrow($result))
+		{
+			$pos = $positions[$row['attach_id']];
+
+			// If someone double-submits a form, the second submission will
+			// appear to have valid attachment IDs even though they were already
+			// consumed by the first submission. To avoid this, match the orphan
+			// states so that the files are excluded from the next submission.
+			// Without this check, the second post will have its
+			// `post_attachments` metadata field set because
+			// `$this->attachment_data` will not be empty, but it *won’t* have
+			// attachments since `submit_post` will not update database records
+			// which were already consumed.
+			if ($row['is_orphan'] == $attachment_data[$pos]['is_orphan'])
 			{
-				$pos = $orphan[$row['attach_id']];
 				$this->attachment_data[$pos] = $row;
 				if (isset($attachment_data[$pos]['attach_comment']))
 				{
 					$this->attachment_data[$pos]['attach_comment'] = $attachment_data[$pos]['attach_comment'];
 				}
-
-				unset($orphan[$row['attach_id']]);
+				unset($positions[$row['attach_id']]);
 			}
-			$db->sql_freeresult($result);
-		}
-
-		if (count($orphan))
-		{
-			trigger_error('NO_ACCESS_ATTACHMENT', E_USER_ERROR);
 		}
+		$db->sql_freeresult($result);
 
 		ksort($this->attachment_data);
+
+		return count($positions) === 0;
 	}
 
 	/**
diff --git a/includes/ucp/ucp_pm_compose.php b/includes/ucp/ucp_pm_compose.php
index 1a12fcc1..4b376896 100644
--- a/includes/ucp/ucp_pm_compose.php
+++ b/includes/ucp/ucp_pm_compose.php
@@ -602,7 +602,10 @@ function compose_pm($id, $mode, $action, $user_folders = array())
 
 	// Always check if the submitted attachment data is valid and belongs to the user.
 	// Further down (especially in submit_post()) we do not check this again.
-	$message_parser->get_submitted_attachment_data();
+	if (!$message_parser->get_submitted_attachment_data())
+	{
+		$error[] = $user->lang('NO_ACCESS_ATTACHMENT');
+	}
 
 	if ($message_attachment && !$submit && !$refresh && !$preview && $action == 'edit')
 	{
diff --git a/language/en/common.php b/language/en/common.php
index fe4f0419..dd83ac17 100644
--- a/language/en/common.php
+++ b/language/en/common.php
@@ -492,7 +492,7 @@ $lang = array_merge($lang, array(
 	),
 	'NOTIFY_ADMIN'				=> 'Please notify the board administrator or webmaster.',
 	'NOTIFY_ADMIN_EMAIL'		=> 'Please notify the board administrator or webmaster: <a href="mailto:%1$s">%1$s</a>',
-	'NO_ACCESS_ATTACHMENT'		=> 'You are not allowed to access this file.',
+	'NO_ACCESS_ATTACHMENT'		=> 'Some attachments are no longer available. Please check and re-attach any missing files.',
 	'NO_ACTION'					=> 'No action specified.',
 	'NO_ADMINISTRATORS'			=> 'There are no administrators.',
 	'NO_AUTH_ADMIN'				=> 'Access to the Administration Control Panel is not allowed as you do not have administrative permissions.',
diff --git a/posting.php b/posting.php
index b8b18b9e..c7437204 100644
--- a/posting.php
+++ b/posting.php
@@ -641,7 +641,10 @@ unset($uninit);
 
 // Always check if the submitted attachment data is valid and belongs to the user.
 // Further down (especially in submit_post()) we do not check this again.
-$message_parser->get_submitted_attachment_data($post_data['poster_id']);
+if (!$message_parser->get_submitted_attachment_data($post_data['poster_id']))
+{
+	$error[] = $language->lang('NO_ACCESS_ATTACHMENT');
+}
 
 if ($post_data['post_attachment'] && !$submit && !$refresh && !$preview && $mode == 'edit')
 {