diff -u b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
--- b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -179,6 +179,20 @@
   protected $configImporter;
 
   /**
+   * A list of unique strings generated by randomStringUnique().
+   *
+   * @var array
+   */
+  protected $uniqueStrings = array();
+
+  /**
+   * A list of unique strings generated by randomStringUnique().
+   *
+   * @var array
+   */
+  protected $uniqueNames = array();
+
+  /**
    * Constructor for Test.
    *
    * @param $test_id
@@ -1189,13 +1203,12 @@
    *
    * @see Drupal\simpletest\TestBase::randomName()
    */
-  public static function randomStringUnique($length = 8) {
-    static $strings = array();
+  public function randomStringUnique($length = 8) {
     do {
-      $string = static::randomString($length);
-    } while (isset($names[$string]));
+      $string = $this->randomString($length);
+    } while (isset($this->uniqueStrings[$string]));
 
-    $strings[$string] = TRUE;
+    $this->uniqueStrings[$string] = TRUE;
     return $string;
   }
 
@@ -1240,13 +1253,12 @@
    *
    * @see Drupal\simpletest\TestBase::randomName()
    */
-  public static function randomNameUnique($length = 8) {
-    static $names = array();
+  public function randomNameUnique($length = 8) {
     do {
-      $name = static::randomName($length);
-    } while (isset($names[$name]));
+      $name = $this->randomName($length);
+    } while (isset($this->uniqueNames[$name]));
 
-    $names[$name] = TRUE;
+    $this->uniqueNames[$name] = TRUE;
     return $name;
   }