diff --git a/src/socketio/base_manager.py b/src/socketio/base_manager.py
index 49e9e98..0d6e1a9 100644
--- a/src/socketio/base_manager.py
+++ b/src/socketio/base_manager.py
@@ -1,7 +1,7 @@
 import itertools
 import logging
 
-from bidict import bidict
+from bidict import bidict, ValueDuplicationError
 
 default_logger = logging.getLogger('socketio')
 
@@ -51,7 +51,11 @@ class BaseManager(object):
     def connect(self, eio_sid, namespace):
         """Register a client connection to a namespace."""
         sid = self.server.eio.generate_id()
-        self.enter_room(sid, namespace, None, eio_sid=eio_sid)
+        try:
+            self.enter_room(sid, namespace, None, eio_sid=eio_sid)
+        except ValueDuplicationError:
+            # already connected
+            return None
         self.enter_room(sid, namespace, sid, eio_sid=eio_sid)
         return sid
 
diff --git a/src/socketio/server.py b/src/socketio/server.py
index 9084120..95c7134 100644
--- a/src/socketio/server.py
+++ b/src/socketio/server.py
@@ -641,6 +641,12 @@ class Server(object):
         """Handle a client connection request."""
         namespace = namespace or '/'
         sid = self.manager.connect(eio_sid, namespace)
+        if sid is None:
+            self._send_packet(eio_sid, self.packet_class(
+                packet.CONNECT_ERROR, data='Unable to connect',
+                namespace=namespace))
+            return
+
         if self.always_connect:
             self._send_packet(eio_sid, self.packet_class(
                 packet.CONNECT, {'sid': sid}, namespace=namespace))
diff --git a/tests/common/test_server.py b/tests/common/test_server.py
index 3325fcb..1bdac11 100644
--- a/tests/common/test_server.py
+++ b/tests/common/test_server.py
@@ -366,6 +366,19 @@ class TestServer(unittest.TestCase):
         handler.assert_called_once_with('1', 'environ')
         s.eio.send.assert_called_once_with('123', '0/foo,{"sid":"1"}')
 
+    def test_handle_connect_namespace_twice(self, eio):
+        s = server.Server()
+        handler = mock.MagicMock()
+        s.on('connect', handler, namespace='/foo')
+        s._handle_eio_connect('123', 'environ')
+        s._handle_eio_message('123', '0/foo,')
+        s._handle_eio_message('123', '0/foo,')
+        assert s.manager.is_connected('1', '/foo')
+        handler.assert_called_once_with('1', 'environ')
+        s.eio.send.assert_any_call('123', '0/foo,{"sid":"1"}')
+        print(s.eio.send.call_args_list)
+        s.eio.send.assert_any_call('123', '4/foo,"Unable to connect"')
+
     def test_handle_connect_always_connect(self, eio):
         s = server.Server(always_connect=True)
         s.manager.initialize = mock.MagicMock()