commit 9317ef39e9f2d8d6f93c8f50f44e70efba46d900
Author: Joel Pittet <joel@pittet.ca>
Date:   Mon Aug 4 14:55:26 2014 -0700

    unique and docs

diff --git a/core/lib/Drupal/Core/Template/Attribute.php b/core/lib/Drupal/Core/Template/Attribute.php
index 1188467..075ef8b 100644
--- a/core/lib/Drupal/Core/Template/Attribute.php
+++ b/core/lib/Drupal/Core/Template/Attribute.php
@@ -119,7 +119,7 @@ public function offsetExists($name) {
   /**
    * Adds argument values by merging them on to array of existing CSS classes.
    *
-   * @param mixed ...
+   * @param string|array ...
    *   CSS classes to add to the class attribute array.
    *
    * @return $this
@@ -139,12 +139,12 @@ public function addClass() {
       // Merge the values passed in from the class value array.
       $classes = array_merge($this->storage['class']->value(), $classes);
       // Filter out any empty values.
-      $classes = array_filter($classes);
+      $classes = array_unique(array_filter($classes));
       $this->storage['class']->exchangeArray($classes);
     }
     else {
       // Filter out any empty values.
-      $classes = array_filter($classes);
+      $classes = array_unique(array_filter($classes));
       $this->offsetSet('class', $classes);
     }
 
@@ -154,7 +154,7 @@ public function addClass() {
   /**
    * Removes argument values from array of existing CSS classes.
    *
-   * @param mixed ...
+   * @param string|array ...
    *   CSS classes to remove from the class attribute array.
    *
    * @return $this
@@ -174,7 +174,7 @@ public function removeClass() {
       // Remove the values passed in from the value array.
       $classes = array_diff($this->storage['class']->value(), $classes);
       // Filter out any empty values.
-      $classes = array_filter($classes);
+      $classes = array_unique(array_filter($classes));
       $this->storage['class']->exchangeArray($classes);
     }
     return $this;