diff --git a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
index d3ba7b3..860cc3f 100644
--- a/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 $test_id
@@ -1179,6 +1193,26 @@ public static function randomString($length = 8) {
   }
 
   /**
+   * Generates a unique random string of ASCII characters of codes 32 to 126.
+   *
+   * @param int $length
+   *   Length of random name to generate.
+   *
+   * @return string
+   *   A unique randomly generated string.
+   *
+   * @see Drupal\simpletest\TestBase::randomName()
+   */
+  public function randomStringUnique($length = 8) {
+    do {
+      $string = $this->randomString($length);
+    } while (isset($this->uniqueStrings[$string]));
+
+    $this->uniqueStrings[$string] = TRUE;
+    return $string;
+  }
+
+  /**
    * Generates a random string containing letters and numbers.
    *
    * The string will always start with a letter. The letters may be upper or
@@ -1209,6 +1243,26 @@ public static function randomName($length = 8) {
   }
 
   /**
+   * Generates a unique random string containing letters and numbers.
+   *
+   * @param int $length
+   *   Length of random name to generate.
+   *
+   * @return string
+   *   A unique randomly generated string.
+   *
+   * @see Drupal\simpletest\TestBase::randomName()
+   */
+  public function randomNameUnique($length = 8) {
+    do {
+      $name = $this->randomName($length);
+    } while (isset($this->uniqueNames[$name]));
+
+    $this->uniqueNames[$name] = TRUE;
+    return $name;
+  }
+
+  /**
    * Generates a random PHP object.
    *
    * @param int $size
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index 8c46872..c906ae6 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -487,7 +487,7 @@ protected function drupalCreateUser(array $permissions = array(), $name = NULL)
 
     // Create a user assigned to that role.
     $edit = array();
-    $edit['name']   = !empty($name) ? $name : $this->randomName();
+    $edit['name']   = !empty($name) ? $name : $this->randomNameUnique();
     $edit['mail']   = $edit['name'] . '@example.com';
     $edit['pass']   = user_password();
     $edit['status'] = 1;
@@ -527,13 +527,13 @@ protected function drupalCreateUser(array $permissions = array(), $name = NULL)
   protected function drupalCreateRole(array $permissions, $rid = NULL, $name = NULL, $weight = NULL) {
     // Generate a random, lowercase machine name if none was passed.
     if (!isset($rid)) {
-      $rid = strtolower($this->randomName(8));
+      $rid = strtolower($this->randomNameUnique(8));
     }
     // Generate a random label.
     if (!isset($name)) {
       // In the role UI role names are trimmed and random string can start or
       // end with a space.
-      $name = trim($this->randomString(8));
+      $name = trim($this->randomStringUnique(8));
     }
 
     // Check the all the permissions strings are valid.