diff --git a/proxy/common/flags.py b/proxy/common/flags.py
index 7f92fd58..12318c97 100644
--- a/proxy/common/flags.py
+++ b/proxy/common/flags.py
@@ -518,7 +518,7 @@ class Flags:
         }
         for plugin_ in plugins:
             if isinstance(plugin_, type):
-                module_name = None
+                module_name = '__main__'
                 klass = plugin_
             else:
                 plugin = text_(plugin_.strip())
@@ -536,7 +536,8 @@ class Flags:
             while next(iterator) is not abc.ABC:
                 pass
             base_klass = next(iterator)
-            p[bytes_(base_klass.__name__)].append(klass)
+            if klass not in p[bytes_(base_klass.__name__)]:
+                p[bytes_(base_klass.__name__)].append(klass)
             logger.info('Loaded plugin %s.%s', module_name, klass.__name__)
         return p
 
diff --git a/tests/common/test_flags.py b/tests/common/test_flags.py
index 2cc10fd0..93774fc3 100644
--- a/tests/common/test_flags.py
+++ b/tests/common/test_flags.py
@@ -10,26 +10,29 @@
 """
 import unittest
 
+from typing import List, Dict
+
 from proxy.common.flags import Flags
 from proxy.http.proxy import HttpProxyPlugin
 from proxy.plugin import CacheResponsesPlugin
 from proxy.plugin import FilterByUpstreamHostPlugin
 
+
 class TestFlags(unittest.TestCase):
-    def assert_plugins(self, expected):
+    def assert_plugins(self, expected: Dict[str, List[type]]) -> None:
         for k in expected:
             self.assertIn(k.encode(), self.flags.plugins)
             for p in expected[k]:
                 self.assertIn(p, self.flags.plugins[k.encode()])
                 self.assertEqual(len([o for o in self.flags.plugins[k.encode()] if o == p]), 1)
 
-    def test_load_plugin_from_bytes(self):
+    def test_load_plugin_from_bytes(self) -> None:
         self.flags = Flags.initialize([], plugins=[
             b'proxy.plugin.CacheResponsesPlugin',
         ])
         self.assert_plugins({'HttpProxyBasePlugin': [CacheResponsesPlugin]})
 
-    def test_load_plugins_from_bytes(self):
+    def test_load_plugins_from_bytes(self) -> None:
         self.flags = Flags.initialize([], plugins=[
             b'proxy.plugin.CacheResponsesPlugin',
             b'proxy.plugin.FilterByUpstreamHostPlugin',
@@ -39,13 +42,13 @@ class TestFlags(unittest.TestCase):
             FilterByUpstreamHostPlugin,
         ]})
 
-    def test_load_plugin_from_args(self):
+    def test_load_plugin_from_args(self) -> None:
         self.flags = Flags.initialize([
             '--plugins', 'proxy.plugin.CacheResponsesPlugin',
         ])
         self.assert_plugins({'HttpProxyBasePlugin': [CacheResponsesPlugin]})
 
-    def test_load_plugins_from_args(self):
+    def test_load_plugins_from_args(self) -> None:
         self.flags = Flags.initialize([
             '--plugins', 'proxy.plugin.CacheResponsesPlugin,proxy.plugin.FilterByUpstreamHostPlugin',
         ])
@@ -54,13 +57,13 @@ class TestFlags(unittest.TestCase):
             FilterByUpstreamHostPlugin,
         ]})
 
-    def test_load_plugin_from_class(self):
+    def test_load_plugin_from_class(self) -> None:
         self.flags = Flags.initialize([], plugins=[
             CacheResponsesPlugin,
         ])
         self.assert_plugins({'HttpProxyBasePlugin': [CacheResponsesPlugin]})
 
-    def test_load_plugins_from_class(self):
+    def test_load_plugins_from_class(self) -> None:
         self.flags = Flags.initialize([], plugins=[
             CacheResponsesPlugin,
             FilterByUpstreamHostPlugin,
@@ -70,7 +73,7 @@ class TestFlags(unittest.TestCase):
             FilterByUpstreamHostPlugin,
         ]})
 
-    def test_load_plugins_from_bytes_and_class(self):
+    def test_load_plugins_from_bytes_and_class(self) -> None:
         self.flags = Flags.initialize([], plugins=[
             CacheResponsesPlugin,
             b'proxy.plugin.FilterByUpstreamHostPlugin',
@@ -80,8 +83,7 @@ class TestFlags(unittest.TestCase):
             FilterByUpstreamHostPlugin,
         ]})
 
-    @unittest.expectedFailure
-    def test_unique_plugin_from_bytes(self):
+    def test_unique_plugin_from_bytes(self) -> None:
         self.flags = Flags.initialize([], plugins=[
             b'proxy.http.proxy.HttpProxyPlugin',
         ])
@@ -89,8 +91,7 @@ class TestFlags(unittest.TestCase):
             HttpProxyPlugin,
         ]})
 
-    @unittest.expectedFailure
-    def test_unique_plugin_from_args(self):
+    def test_unique_plugin_from_args(self) -> None:
         self.flags = Flags.initialize([
             '--plugins', 'proxy.http.proxy.HttpProxyPlugin',
         ])
@@ -98,8 +99,7 @@ class TestFlags(unittest.TestCase):
             HttpProxyPlugin,
         ]})
 
-    @unittest.expectedFailure
-    def test_unique_plugin_from_class(self):
+    def test_unique_plugin_from_class(self) -> None:
         self.flags = Flags.initialize([], plugins=[
             HttpProxyPlugin,
         ])
@@ -107,5 +107,6 @@ class TestFlags(unittest.TestCase):
             HttpProxyPlugin,
         ]})
 
+
 if __name__ == '__main__':
     unittest.main()