Fix plugins running twice in autoreload mode

Fixes #2817

pelican/plugins/_utils.py

@@ -53,6 +53,9 @@ def load_legacy_plugin(plugin, plugin_paths):
     if spec is None:
         raise ImportError('Cannot import plugin `{}`'.format(plugin))
     else:
+        # Avoid loading the same plugin twice
+        if spec.name in sys.modules:
+            return sys.modules[spec.name]
         # create module object from spec
         mod = importlib.util.module_from_spec(spec)
         # place it into sys.modules cache

pelican/tests/test_plugins.py

@@ -131,6 +131,17 @@ class PluginTest(unittest.TestCase):
                  'normal subpackage plugin'},
                 get_plugin_names(plugins))
 
+            # ensure normal plugins are loaded only once
+            SETTINGS = {
+                'PLUGINS': ['normal_plugin'],
+                'PLUGIN_PATHS': [self._NORMAL_PLUGIN_FOLDER],
+            }
+            plugins = load_plugins(SETTINGS)
+            for plugin in load_plugins(SETTINGS):
+                # The second load_plugins() should return the same plugin
+                # objects as the first one
+                self.assertIn(plugin, plugins)
+
             # namespace plugin short
             SETTINGS = {
                 'PLUGINS': ['ns_plugin']