62546 do not add duplicate css classes

Fix https://core.trac.wordpress.org/ticket/62546

src/wp-admin/includes/menu.php

@@ -203,10 +203,14 @@
  * @return string The string with the CSS class added.
  */
 function add_cssclass( $class_to_add, $classes ) {
-	if ( empty( $classes ) ) {
+	if ( '' === $classes ) {
 		return $class_to_add;
 	}
 
+	if ( preg_match( '/(?>^|\s)' . preg_quote( $class_to_add, '/' ) . '(?>\s|$)/', $classes ) ) {
+		return $classes;
+	}
+
 	return $classes . ' ' . $class_to_add;
 }
 

tests/phpunit/tests/menu/menu-functions.php

@@ -0,0 +1,44 @@
+<?php
+
+/**
+ * @group menu
+ */
+class Tests_Menu_Functions extends WP_UnitTestCase {
+	/**
+	 * @dataProvider data_add_cssclass
+	 *
+	 * @covers add_cssclass
+	 */
+	public function test_add_cssclass( $input, $expected ) {
+		$this->assertSame( $expected, add_cssclass( 'menu-top-first', $input ) );
+	}
+
+	public function data_add_cssclass() {
+		return array(
+			'class should not be added one'   => array(
+				'input'    => 'menu-top menu-top-first menu-icon-dashboard',
+				'expected' => 'menu-top menu-top-first menu-icon-dashboard',
+			),
+			'class should not be added two'   => array(
+				'input'    => 'menu-top menu-top-first',
+				'expected' => 'menu-top menu-top-first',
+			),
+			'class should not be added three' => array(
+				'input'    => 'menu-top-first',
+				'expected' => 'menu-top-first',
+			),
+			'class should not be added four'  => array(
+				'input'    => 'menu-top-first menu-icon-dashboard',
+				'expected' => 'menu-top-first menu-icon-dashboard',
+			),
+			'class should be added one'       => array(
+				'input'    => 'menu-top menu-top-first-foo menu-icon-dashboard',
+				'expected' => 'menu-top menu-top-first-foo menu-icon-dashboard menu-top-first',
+			),
+			'class should be added two'       => array(
+				'input'    => 'menu-top foo-menu-top-first menu-icon-dashboard',
+				'expected' => 'menu-top foo-menu-top-first menu-icon-dashboard menu-top-first',
+			),
+		);
+	}
+}