diff --git a/dsd_pythonanywhere/deploy_messages.py b/dsd_pythonanywhere/deploy_messages.py
index 9216b4d..9b94774 100644
--- a/dsd_pythonanywhere/deploy_messages.py
+++ b/dsd_pythonanywhere/deploy_messages.py
@@ -30,6 +30,16 @@
   $ ...
 """
 
+settings_found_pythonanywhere = """
+A PythonAnywhere-specific settings block was already found in settings.py.
+That block will be replaced with a fresh copy.
+"""
+
+settings_cant_overwrite_pythonanywhere = """
+Can't overwrite the existing PythonAnywhere-specific settings block. Please
+remove it from settings.py manually and run the deploy command again.
+"""
+
 
 # --- Dynamic strings ---
 # These need to be generated in functions, to display information that's determined as
diff --git a/dsd_pythonanywhere/platform_deployer.py b/dsd_pythonanywhere/platform_deployer.py
index 4aa8b5d..1361f8f 100644
--- a/dsd_pythonanywhere/platform_deployer.py
+++ b/dsd_pythonanywhere/platform_deployer.py
@@ -91,6 +91,17 @@ def _validate_platform(self) -> None:
                 "Please verify your API_USER and API_TOKEN are correct."
             )
 
+        self._check_pythonanywhere_settings()
+
+    def _check_pythonanywhere_settings(self) -> None:
+        """Check to see if a PythonAnywhere settings block already exists."""
+        plugin_utils.check_settings(
+            "PythonAnywhere",
+            "# PythonAnywhere settings.",
+            platform_msgs.settings_found_pythonanywhere,
+            platform_msgs.settings_cant_overwrite_pythonanywhere,
+        )
+
     def _get_origin_url(self) -> str:
         """Get the git remote origin URL."""
         origin_url = (
diff --git a/tests/unit_tests/test_platform_deployer.py b/tests/unit_tests/test_platform_deployer.py
index a5a36b2..77532a2 100644
--- a/tests/unit_tests/test_platform_deployer.py
+++ b/tests/unit_tests/test_platform_deployer.py
@@ -11,6 +11,14 @@
 )
 
 
+@pytest.fixture
+def settings_path(tmp_path: Path):
+    settings_path = tmp_path / "settings.py"
+    settings_content = "# Existing settings"
+    settings_path.write_text(settings_content)
+    yield settings_path
+
+
 def test_modify_gitignore(tmp_path: Path, monkeypatch):
     """_modify_gitignore adds patterns correctly."""
     deployer = PlatformDeployer()
@@ -40,12 +48,8 @@ def test_modify_gitignore(tmp_path: Path, monkeypatch):
     assert contents == original_contents
 
 
-def test_modify_settings(tmp_path: Path, monkeypatch):
+def test_modify_settings(settings_path, monkeypatch):
     """_modify_settings modifies settings.py as expected."""
-    settings_path = tmp_path / "settings.py"
-    settings_content = "# Existing settings"
-    settings_path.write_text(settings_content)
-
     monkeypatch.setenv("API_USER", "testuser")
     deployer = PlatformDeployer()
     monkeypatch.setattr(dsd_config, "settings_path", settings_path)
@@ -175,11 +179,12 @@ def test_validate_platform_api_connection_fails(monkeypatch, mocker):
         deployer._validate_platform()
 
 
-def test_validate_platform_success(monkeypatch, mocker):
+def test_validate_platform_success(settings_path, monkeypatch, mocker):
     """_validate_platform succeeds with valid credentials."""
     monkeypatch.setenv("API_USER", "test_user")
     monkeypatch.setenv("API_TOKEN", "test_token")
     monkeypatch.setattr(dsd_config, "automate_all", True)
+    monkeypatch.setattr(dsd_config, "settings_path", settings_path)
 
     deployer = PlatformDeployer()
     mock_request = mocker.patch.object(deployer.client, "request")
@@ -200,3 +205,24 @@ def test_validate_platform_skipped_without_automate_all(monkeypatch):
     deployer = PlatformDeployer()
     # Should not raise any exception even without credentials
     deployer._validate_platform()
+
+
+def test_validate_platform_removes_existing_settings(settings_path, monkeypatch, mocker):
+    """_validate_platform replaces settings block, preventing stacking copies."""
+    monkeypatch.setenv("API_USER", "test_user")
+    monkeypatch.setenv("API_TOKEN", "test_token")
+    monkeypatch.setattr(dsd_config, "automate_all", True)
+    monkeypatch.setattr(dsd_config, "settings_path", settings_path)
+    monkeypatch.setattr(dsd_config, "stdout", sys.stdout)
+    monkeypatch.setattr(dsd_config, "unit_testing", True)
+
+    deployer = PlatformDeployer()
+    mock_request = mocker.patch.object(deployer.client, "request")
+    mock_response = mocker.Mock()
+    mock_response.ok = True
+    mock_request.return_value = mock_response
+
+    settings_path.write_text("# PythonAnywhere settings.")
+    deployer._validate_platform()
+    modified_content = settings_path.read_text()
+    assert modified_content.count("# PythonAnywhere settings.") == 0