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function wp_allow_comment( $commentdata, $wp_error = false ) {
	global $wpdb;

	/*
	 * Simple duplicate check.
	 * expected_slashed ($comment_post_ID, $comment_author, $comment_author_email, $comment_content)
	 */
	$dupe = $wpdb->prepare(
		"SELECT comment_ID FROM $wpdb->comments WHERE comment_post_ID = %d AND comment_parent = %s AND comment_approved != 'trash' AND ( comment_author = %s ",
		wp_unslash( $commentdata['comment_post_ID'] ),
		wp_unslash( $commentdata['comment_parent'] ),
		wp_unslash( $commentdata['comment_author'] )
	);
	if ( $commentdata['comment_author_email'] ) {
		$dupe .= $wpdb->prepare(
			'AND comment_author_email = %s ',
			wp_unslash( $commentdata['comment_author_email'] )
		);
	}
	$dupe .= $wpdb->prepare(
		') AND comment_content = %s LIMIT 1',
		wp_unslash( $commentdata['comment_content'] )
	);

	$dupe_id = $wpdb->get_var( $dupe );

	/**
	 * Filters the ID, if any, of the duplicate comment found when creating a new comment.
	 *
	 * Return an empty value from this filter to allow what WP considers a duplicate comment.
	 *
	 * @since 4.4.0
	 *
	 * @param int   $dupe_id     ID of the comment identified as a duplicate.
	 * @param array $commentdata Data for the comment being created.
	 */
	$dupe_id = apply_filters( 'duplicate_comment_id', $dupe_id, $commentdata );

	if ( $dupe_id ) {
		/**
		 * Fires immediately after a duplicate comment is detected.
		 *
		 * @since 3.0.0
		 *
		 * @param array $commentdata Comment data.
		 */
		do_action( 'comment_duplicate_trigger', $commentdata );

		/**
		 * Filters duplicate comment error message.
		 *
		 * @since 5.2.0
		 *
		 * @param string $comment_duplicate_message Duplicate comment error message.
		 */
		$comment_duplicate_message = apply_filters( 'comment_duplicate_message', __( 'Duplicate comment detected; it looks as though you’ve already said that!' ) );

		if ( $wp_error ) {
			return new WP_Error( 'comment_duplicate', $comment_duplicate_message, 409 );
		} else {
			if ( wp_doing_ajax() ) {
				die( $comment_duplicate_message );
			}

			wp_die( $comment_duplicate_message, 409 );
		}
	}

	/**
	 * Fires immediately before a comment is marked approved.
	 *
	 * Allows checking for comment flooding.
	 *
	 * @since 2.3.0
	 * @since 4.7.0 The `$avoid_die` parameter was added.
	 * @since 5.5.0 The `$avoid_die` parameter was renamed to `$wp_error`.
	 *
	 * @param string $comment_author_ip    Comment author's IP address.
	 * @param string $comment_author_email Comment author's email.
	 * @param string $comment_date_gmt     GMT date the comment was posted.
	 * @param bool   $wp_error             Whether to return a WP_Error object instead of executing
	 *                                     wp_die() or die() if a comment flood is occurring.
	 */
	do_action(
		'check_comment_flood',
		$commentdata['comment_author_IP'],
		$commentdata['comment_author_email'],
		$commentdata['comment_date_gmt'],
		$wp_error
	);

	/**
	 * Filters whether a comment is part of a comment flood.
	 *
	 * The default check is wp_check_comment_flood(). See check_comment_flood_db().
	 *
	 * @since 4.7.0
	 * @since 5.5.0 The `$avoid_die` parameter was renamed to `$wp_error`.
	 *
	 * @param bool   $is_flood             Is a comment flooding occurring? Default false.
	 * @param string $comment_author_ip    Comment author's IP address.
	 * @param string $comment_author_email Comment author's email.
	 * @param string $comment_date_gmt     GMT date the comment was posted.
	 * @param bool   $wp_error             Whether to return a WP_Error object instead of executing
	 *                                     wp_die() or die() if a comment flood is occurring.
	 */
	$is_flood = apply_filters(
		'wp_is_comment_flood',
		false,
		$commentdata['comment_author_IP'],
		$commentdata['comment_author_email'],
		$commentdata['comment_date_gmt'],
		$wp_error
	);

	if ( $is_flood ) {
		/** This filter is documented in wp-includes/comment-template.php */
		$comment_flood_message = apply_filters( 'comment_flood_message', __( 'You are posting comments too quickly. Slow down.' ) );

		return new WP_Error( 'comment_flood', $comment_flood_message, 429 );
	}

	return wp_check_comment_data( $commentdata );
}