[ Dy2Static ] fix the outputs of net is x,x (#44313)

* fix the outputs of net is x,x

* add unittest for duplicate output

* fix

python/paddle/fluid/dygraph/dygraph_to_static/partial_program.py

@@ -441,18 +441,26 @@ def _prepare(self, inputs):
                 continue
             input_vars.append(var)
 
+        # mapping from name(string) -> VarBase
+        out_varbase_map = {}
+
         def create_out(var_id):
             var = self._outputs[var_id]
             assert isinstance(var, framework.Variable)
             var_desc = var.desc
             varbase = None
+
+            if var_desc.name() in out_varbase_map:
+                return out_varbase_map[var_desc.name()]
+
             if not framework._in_eager_mode_:
                 var_base = core.VarBase(var_desc.dtype(), var_desc.shape(),
                                         var_desc.name(), var_desc.type(), False)
             else:
                 var_base = core.eager.Tensor(var_desc.dtype(), var_desc.shape(),
                                              var_desc.name(), var_desc.type(),
                                              False)
+            out_varbase_map[var_desc.name()] = var_base
             return var_base
 
         # Create VarBase to receive output data.

python/paddle/fluid/tests/unittests/dygraph_to_static/test_duplicate_output.py

@@ -0,0 +1,65 @@
+#   Copyright (c) 2019 PaddlePaddle Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import print_function
+
+import numpy as np
+import unittest
+
+import paddle
+
+np.random.seed(1)
+
+if paddle.fluid.is_compiled_with_cuda():
+    place = paddle.fluid.CUDAPlace(0)
+else:
+    place = paddle.fluid.CPUPlace()
+
+
+class SimpleNet(paddle.nn.Layer):
+
+    def __init__(self):
+        super().__init__()
+        self._linear = paddle.nn.Linear(1, 1)
+
+    def forward(self, x):
+        """ forward with duplicate outputs.
+        """
+        x = self._linear(x)
+        return x, x
+
+
+class TestDuplicateOutput(unittest.TestCase):
+    """
+    TestCase for the transformation from control flow `if/else`
+    dependent on tensor in Dygraph into Static `fluid.layers.cond`.
+    """
+
+    def setUp(self):
+        self.net = paddle.jit.to_static(SimpleNet())
+        self.x = paddle.to_tensor([1.0])
+
+    def _run_static(self):
+        loss0, loss1 = self.net(self.x)
+        loss0.backward()
+        param = self.net.parameters()
+        self.assertEqual(param[0].grad.numpy(), 1.0)
+
+    def test_ast_to_func(self):
+        self._run_static()
+
+
+if __name__ == '__main__':
+    with paddle.fluid.framework._test_eager_guard():
+        unittest.main()