diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7c4255af8e..6a8cf96c8e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -164,6 +164,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed distributed setting and `ddp_cpu` only with `num_processes>1` ([#5297](https://github.com/PyTorchLightning/pytorch-lightning/pull/5297))
 
 
+- Fixed an error when logging a progress bar metric with a reserved name ([#5620](https://github.com/PyTorchLightning/pytorch-lightning/pull/5620))
+
+
 - Fixed the saved filename in `ModelCheckpoint` when it already exists ([#4861](https://github.com/PyTorchLightning/pytorch-lightning/pull/4861))
 
 
diff --git a/pytorch_lightning/trainer/properties.py b/pytorch_lightning/trainer/properties.py
index 760a621db6..aca37802ca 100644
--- a/pytorch_lightning/trainer/properties.py
+++ b/pytorch_lightning/trainer/properties.py
@@ -14,11 +14,15 @@
 import inspect
 import os
 from abc import ABC
-from argparse import ArgumentParser, Namespace
+from argparse import ArgumentParser
+from argparse import Namespace
 from typing import cast, List, Optional, Type, TypeVar, Union
 
 from pytorch_lightning.accelerators.legacy.accelerator import Accelerator
-from pytorch_lightning.callbacks import Callback, EarlyStopping, ModelCheckpoint, ProgressBarBase
+from pytorch_lightning.callbacks import Callback
+from pytorch_lightning.callbacks import EarlyStopping
+from pytorch_lightning.callbacks import ModelCheckpoint
+from pytorch_lightning.callbacks import ProgressBarBase
 from pytorch_lightning.core.lightning import LightningModule
 from pytorch_lightning.loggers.base import LightningLoggerBase
 from pytorch_lightning.loggers.tensorboard import TensorBoardLogger
@@ -26,13 +30,15 @@ from pytorch_lightning.trainer.connectors.checkpoint_connector import Checkpoint
 from pytorch_lightning.trainer.connectors.logger_connector import LoggerConnector
 from pytorch_lightning.trainer.connectors.model_connector import ModelConnector
 from pytorch_lightning.trainer.states import TrainerState
-from pytorch_lightning.utilities import _HOROVOD_AVAILABLE, _TPU_AVAILABLE, DeviceType, DistributedType
-from pytorch_lightning.utilities.argparse import (
-    add_argparse_args,
-    from_argparse_args,
-    parse_argparser,
-    parse_env_variables,
-)
+from pytorch_lightning.utilities import _HOROVOD_AVAILABLE
+from pytorch_lightning.utilities import _TPU_AVAILABLE
+from pytorch_lightning.utilities import DeviceType
+from pytorch_lightning.utilities import DistributedType
+from pytorch_lightning.utilities import rank_zero_warn
+from pytorch_lightning.utilities.argparse import add_argparse_args
+from pytorch_lightning.utilities.argparse import from_argparse_args
+from pytorch_lightning.utilities.argparse import parse_argparser
+from pytorch_lightning.utilities.argparse import parse_env_variables
 from pytorch_lightning.utilities.cloud_io import get_filesystem
 from pytorch_lightning.utilities.model_helpers import is_overridden
 
@@ -193,7 +199,20 @@ class TrainerProperties(ABC):
         """ Read-only for progress bar metrics. """
         ref_model = self.get_model()
         ref_model = cast(LightningModule, ref_model)
-        return dict(**ref_model.get_progress_bar_dict(), **self.logger_connector.progress_bar_metrics)
+
+        standard_metrics = ref_model.get_progress_bar_dict()
+        logged_metrics = self.progress_bar_metrics
+        duplicates = list(standard_metrics.keys() & logged_metrics.keys())
+        if duplicates:
+            rank_zero_warn(
+                f"The progress bar already tracks a metric with the name(s) '{', '.join(duplicates)}' and"
+                f" `self.log('{duplicates[0]}', ..., prog_bar=True)` will overwrite this value. "
+                f" If this is undesired, change the name or override `get_progress_bar_dict()`"
+                f" in `LightingModule`.", UserWarning
+            )
+        all_metrics = dict(**standard_metrics)
+        all_metrics.update(**logged_metrics)
+        return all_metrics
 
     @property
     def disable_validation(self) -> bool:
diff --git a/tests/accelerators/legacy/test_multi_nodes_gpu.py b/tests/accelerators/legacy/test_multi_nodes_gpu.py
index f17ac42fcb..d9387df2b9 100644
--- a/tests/accelerators/legacy/test_multi_nodes_gpu.py
+++ b/tests/accelerators/legacy/test_multi_nodes_gpu.py
@@ -26,7 +26,9 @@ from pytorch_lightning import Trainer
 from tests.base.boring_model import BoringModel  # noqa: E402
 
 
-@pytest.mark.skipif(not os.getenv("PL_RUNNING_SPECIAL_TESTS", '0') == '1', reason="test should be run outside of pytest")
+@pytest.mark.skipif(
+    not os.getenv("PL_RUNNING_SPECIAL_TESTS", '0') == '1', reason="test should be run outside of pytest"
+)
 def test_logging_sync_dist_true_ddp(tmpdir):
     """
     Tests to ensure that the sync_dist flag works with CPU (should just return the original value)
@@ -62,7 +64,9 @@ def test_logging_sync_dist_true_ddp(tmpdir):
     assert trainer.logged_metrics['bar'] == fake_result
 
 
-@pytest.mark.skipif(not os.getenv("PL_RUNNING_SPECIAL_TESTS", '0') == '1', reason="test should be run outside of pytest")
+@pytest.mark.skipif(
+    not os.getenv("PL_RUNNING_SPECIAL_TESTS", '0') == '1', reason="test should be run outside of pytest"
+)
 def test__validation_step__log(tmpdir):
     """
     Tests that validation_step can log
diff --git a/tests/trainer/logging_/test_logger_connector.py b/tests/trainer/logging_/test_logger_connector.py
index 106a299e43..ffdaea8c52 100644
--- a/tests/trainer/logging_/test_logger_connector.py
+++ b/tests/trainer/logging_/test_logger_connector.py
@@ -28,7 +28,8 @@ from pytorch_lightning.trainer import Trainer
 from pytorch_lightning.trainer.connectors.logger_connector.callback_hook_validator import CallbackHookNameValidator
 from pytorch_lightning.trainer.connectors.logger_connector.metrics_holder import MetricsHolder
 from pytorch_lightning.utilities.exceptions import MisconfigurationException
-from tests.base.boring_model import BoringModel, RandomDataset
+from tests.base.boring_model import BoringModel
+from tests.base.boring_model import RandomDataset
 
 
 def decorator_with_arguments(fx_name: str = '', hook_fx_name: str = None) -> Callable:
@@ -454,3 +455,21 @@ def test_metrics_holder(to_float, tmpdir):
     assert excepted_function(metrics["x"])
     assert excepted_function(metrics["y"])
     assert excepted_function(metrics["z"])
+
+
+def test_logging_to_progress_bar_with_reserved_key(tmpdir):
+    """ Test that logging a metric with a reserved name to the progress bar raises a warning. """
+    class TestModel(BoringModel):
+
+        def training_step(self, *args, **kwargs):
+            output = super().training_step(*args, **kwargs)
+            self.log("loss", output["loss"], prog_bar=True)
+            return output
+
+    model = TestModel()
+    trainer = Trainer(
+        default_root_dir=tmpdir,
+        max_steps=2,
+    )
+    with pytest.warns(UserWarning, match="The progress bar already tracks a metric with the .* 'loss'"):
+        trainer.fit(model)
diff --git a/tests/utilities/test_xla_device_utils.py b/tests/utilities/test_xla_device_utils.py
index dcafa85092..471792da9c 100644
--- a/tests/utilities/test_xla_device_utils.py
+++ b/tests/utilities/test_xla_device_utils.py
@@ -16,13 +16,13 @@ from unittest.mock import patch
 
 import pytest
 
-import pytorch_lightning.utilities.xla_device as xla_utils
+import pytorch_lightning.utilities.xla_device_utils as xla_utils
 from pytorch_lightning.utilities import _TPU_AVAILABLE
 from pytorch_lightning.utilities import _XLA_AVAILABLE
 from tests.base.develop_utils import pl_multi_process_test
 
 
-@pytest.mark.skipif(XLA_AVAILABLE, reason="test requires torch_xla to be absent")
+@pytest.mark.skipif(_XLA_AVAILABLE, reason="test requires torch_xla to be absent")
 def test_tpu_device_absence():
     """Check tpu_device_exists returns None when torch_xla is not available"""
     assert xla_utils.XLADeviceUtils.tpu_device_exists() is None