class_docstr: Test that mutators cannot have duplicate names.
class_docstr_tok: ["Test","that","mutators","can","not","have","duplicate","names","."]
code: def test_uniqueness(self):
      from parlai.core.mutators import register_mutator, Mutator
  
      @register_mutator("test_unique_mutator")
      class Mutator1(Mutator):
          pass
  
      # don't freak out if we accidentally register the exact same class twice
      register_mutator("test_unique_mutator")(Mutator1)
  
      # but do demand uniqueness
      with self.assertRaises(NameError):
  
          @register_mutator("test_unique_mutator")
          class Mutator2(Mutator):
              pass
  
imports: import unittest
  from parlai.core.params import ParlaiParser
  from parlai.core.opt import Opt
  from parlai.core.teachers import create_task_agent_from_taskname
  from parlai.core.message import Message
label_desc: Use image node_id 1 for calling the TestUniqueness obj's underlying member method code with example usage: obj.test_uniqueness() without return types