Emit an error when entry points are duplicated

This avoids flake8 silently ignoring one of the plugins and
fixes https://gitlab.com/pycqa/flake8/-/issues/634.

src/flake8/exceptions.py

@@ -1,6 +1,10 @@
 """Exception classes for all of Flake8."""
 from typing import Dict
 
+if False:  # `typing.TYPE_CHECKING` was introduced in 3.5.2
+    from flake8.plugins.manager import Plugin
+    from flake8._compat import importlib_metadata
+
 
 class Flake8Exception(Exception):
     """Plain Flake8 exception."""
@@ -14,6 +18,33 @@ class ExecutionError(Flake8Exception):
     """Exception raised during execution of Flake8."""
 
 
+class DuplicatePluginEntryPoint(Flake8Exception):
+    """Exception raised when a plugin entry point is already taken."""
+
+    FORMAT = (
+        'Plugin entry point "%(entry_point)s" for "%(new)s" already taken by '
+        '"%(existing)s"'
+    )
+
+    def __init__(self, entry_point, existing_plugin):
+        # type: (importlib_metadata.EntryPoint, Plugin) -> None
+        """Initialize our DuplicatePluginEntryPoint exception."""
+        self.entry_point = entry_point
+        self.existing_plugin = existing_plugin
+        super(DuplicatePluginEntryPoint, self).__init__(
+            entry_point,
+            existing_plugin,
+        )
+
+    def __str__(self):  # type: () -> str
+        """Format our exception message."""
+        return self.FORMAT % {
+            "entry_point": self.entry_point.name,
+            "new": self.entry_point.value,
+            "existing": self.existing_plugin.entry_point.value,
+        }
+
+
 class FailedToLoadPlugin(Flake8Exception):
     """Exception raised when a plugin fails to load."""
 

src/flake8/plugins/manager.py

@@ -273,6 +273,12 @@ class PluginManager(object):  # pylint: disable=too-few-public-methods
             Is this a repo-local plugin?
         """
         name = entry_point.name
+
+        if name in self.plugins:
+            raise exceptions.DuplicatePluginEntryPoint(
+                entry_point, self.plugins[name],
+            )
+
         self.plugins[name] = Plugin(name, entry_point, local=local)
         self.names.append(name)
         LOG.debug('Loaded %r for plugin "%s".', self.plugins[name], name)

tests/fixtures/config_files/local-plugin-duplicate-entry-point.ini

@@ -0,0 +1,4 @@
+[flake8:local-plugins]
+extension =
+  XE = aplugin:ExtensionTestPluginA
+  XE = aplugin:ExtensionTestPluginB

tests/integration/test_plugins.py

@@ -1,9 +1,13 @@
 """Integration tests for plugin loading."""
+import pytest
+
 from flake8.main import application
+from flake8 import exceptions
 
 
 LOCAL_PLUGIN_CONFIG = 'tests/fixtures/config_files/local-plugin.ini'
 LOCAL_PLUGIN_PATH_CONFIG = 'tests/fixtures/config_files/local-plugin-path.ini'
+LOCAL_PLUGIN_DUPLICATE_CONFIG = 'tests/fixtures/config_files/local-plugin-duplicate-entry-point.ini'
 
 
 class ExtensionTestPlugin(object):
@@ -61,3 +65,10 @@ def test_enable_local_plugin_at_non_installed_path():
     app.initialize(['flake8', '--config', LOCAL_PLUGIN_PATH_CONFIG])
 
     assert app.check_plugins['XE'].plugin.name == 'ExtensionTestPlugin2'
+
+
+def test_reject_local_plugins_with_duplicate_entry_point():
+    """Reject duplicate entry points in local-plugins config section."""
+    with pytest.raises(exceptions.DuplicatePluginEntryPoint):
+        app = application.Application()
+        app.initialize(['flake8', '--config', LOCAL_PLUGIN_DUPLICATE_CONFIG])