code: def test_duplicate_outputs(self):
      x = chainer.Variable(np.array(0, np.float32))
      y = chainer.functions.identity(x)
      y.grad = np.array(3, np.float32)
      with testing.assert_warns(RuntimeWarning):
          chainer.backward([y, y])
      # 6 might be expected, but y.grad is used only once
      testing.assert_allclose(x.grad, np.array(3, np.float32))
  
imports: import unittest
  import mock
  import numpy
  import pytest
  import chainer
  from chainer.backends import cuda
  from chainer import testing
  from chainer.testing import attr
  import chainerx
label_desc: Use image node_id 11 for calling the TestBackward obj's underlying member method code with example usage: obj.test_duplicate_outputs() without return types