diff --git a/CHANGELOG.md b/CHANGELOG.md
index 22113b9d6d..ddaf4288a0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -175,6 +175,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Added `on_load_checkpoint` and `on_save_checkpoint` hooks to the `PrecisionPlugin` base class ([#7831](https://github.com/PyTorchLightning/pytorch-lightning/pull/7831))
 
 
+- `LightningCLI` now aborts with a clearer message if config already exists and disables save config during `fast_dev_run`([#7963](https://github.com/PyTorchLightning/pytorch-lightning/pull/7963))
+
+
 ### Deprecated
 
 
diff --git a/pytorch_lightning/utilities/cli.py b/pytorch_lightning/utilities/cli.py
index 3fae52cde1..aed9de3355 100644
--- a/pytorch_lightning/utilities/cli.py
+++ b/pytorch_lightning/utilities/cli.py
@@ -75,7 +75,11 @@ class LightningArgumentParser(ArgumentParser):
 
 
 class SaveConfigCallback(Callback):
-    """Saves a LightningCLI config to the log_dir when training starts"""
+    """Saves a LightningCLI config to the log_dir when training starts
+
+    Raises:
+        RuntimeError: If the config file already exists in the directory to avoid overwriting a previous run
+    """
 
     def __init__(
         self,
@@ -90,6 +94,11 @@ class SaveConfigCallback(Callback):
     def on_train_start(self, trainer: Trainer, pl_module: LightningModule) -> None:
         log_dir = trainer.log_dir or trainer.default_root_dir
         config_path = os.path.join(log_dir, self.config_filename)
+        if os.path.isfile(config_path):
+            raise RuntimeError(
+                f'{self.__class__.__name__} expected {config_path} to not exist. '
+                'Aborting to avoid overwriting results of a previous run.'
+            )
         self.parser.save(self.config, config_path, skip_none=False)
 
 
@@ -231,7 +240,7 @@ class LightningCLI:
                 self.config_init['trainer']['callbacks'].extend(self.trainer_defaults['callbacks'])
             else:
                 self.config_init['trainer']['callbacks'].append(self.trainer_defaults['callbacks'])
-        if self.save_config_callback is not None:
+        if self.save_config_callback and not self.config_init['trainer']['fast_dev_run']:
             config_callback = self.save_config_callback(self.parser, self.config, self.save_config_filename)
             self.config_init['trainer']['callbacks'].append(config_callback)
         self.trainer = self.trainer_class(**self.config_init['trainer'])
diff --git a/tests/utilities/test_cli.py b/tests/utilities/test_cli.py
index e1623b15c2..40ddbf9b16 100644
--- a/tests/utilities/test_cli.py
+++ b/tests/utilities/test_cli.py
@@ -328,6 +328,31 @@ def test_lightning_cli_args(tmpdir):
     assert config['trainer'] == cli.config['trainer']
 
 
+def test_lightning_cli_save_config_cases(tmpdir):
+
+    config_path = tmpdir / 'config.yaml'
+    cli_args = [
+        f'--trainer.default_root_dir={tmpdir}',
+        '--trainer.logger=False',
+        '--trainer.fast_dev_run=1',
+    ]
+
+    # With fast_dev_run!=False config should not be saved
+    with mock.patch('sys.argv', ['any.py'] + cli_args):
+        LightningCLI(BoringModel)
+    assert not os.path.isfile(config_path)
+
+    # With fast_dev_run==False config should be saved
+    cli_args[-1] = '--trainer.max_epochs=1'
+    with mock.patch('sys.argv', ['any.py'] + cli_args):
+        LightningCLI(BoringModel)
+    assert os.path.isfile(config_path)
+
+    # If run again on same directory exception should be raised since config file already exists
+    with mock.patch('sys.argv', ['any.py'] + cli_args), pytest.raises(RuntimeError):
+        LightningCLI(BoringModel)
+
+
 def test_lightning_cli_config_and_subclass_mode(tmpdir):
 
     config = dict(