public function testUpdateFieldSetDoesntAddTwice()
 {
     $fs = new FieldList();
     $t = new SecurityToken();
     $t->updateFieldSet($fs);
     // first
     $t->updateFieldSet($fs);
     // second
     $f = $fs->dataFieldByName($t->getName());
     $this->assertInstanceOf('HiddenField', $f);
     $this->assertEquals(1, $fs->Count());
 }