diff --git a/ccserver/ccserver.py b/ccserver/ccserver.py
index 958e6f25e77dd74399ec14c2512ad5a019489f9d..de9b7596338afe27bf26e1510517939804bcf47a 100644
--- a/ccserver/ccserver.py
+++ b/ccserver/ccserver.py
@@ -7,6 +7,7 @@ from sjrpc.server import SimpleSslRpcServer
 from handlers import WelcomeHandler
 from conf import CCConf
 from client import CCClient
+from exceptions import AlreadyRegistered
 
 class CCServer(object):
     '''
@@ -70,8 +71,12 @@ class CCServer(object):
         :param connection: connection to register
         :param tags: tags to add for the client
         '''
-        
-        self._connected[login] = CCClient(login, role, self, connection)
+
+        if login in self._connected:
+            raise AlreadyRegistered('A client is already connected with this '
+                                    'account.')
+        else:
+            self._connected[login] = CCClient(login, role, self, connection)
 
     def unregister(self, connection):
         '''
diff --git a/ccserver/exceptions.py b/ccserver/exceptions.py
new file mode 100644
index 0000000000000000000000000000000000000000..c05d18147032c597f4185abb7e5fbca774a0d6ad
--- /dev/null
+++ b/ccserver/exceptions.py
@@ -0,0 +1,9 @@
+#!/usr/bin/env python
+#coding=utf8
+
+
+class AlreadyRegistered(Exception):
+    pass
+
+class AuthenticationError(Exception):
+    pass
diff --git a/ccserver/handlers.py b/ccserver/handlers.py
index b34fe7c6084c70c82902500a44c43c764c465c16..d3fc75cd16c8c476d114f716d9c67efcd898dfa0 100644
--- a/ccserver/handlers.py
+++ b/ccserver/handlers.py
@@ -6,6 +6,7 @@ import logging
 from sjrpc.utils import RpcHandler, pure
 from tql import TqlQuery
 from conf import CCConf
+from exceptions import AlreadyRegistered, AuthenticationError
 
 def listed(func):
     func.__listed__ = True
@@ -218,10 +219,6 @@ class ClientHandler(OnlineCCHandler):
         client = self._server.get_connection(login)
         return client.connection.call(command, *args, **kwargs)
 
-class AuthenticationError(Exception):
-    pass
-
-
 class WelcomeHandler(CCHandler):
     '''
     Default handler used on client connections of the server.
@@ -248,8 +245,12 @@ class WelcomeHandler(CCHandler):
             logging.info('New authentication from %s: failure' % login)
             raise AuthenticationError('Authentication failure')
         else:
-            # If authentication is a success, ask tags to the server:
-            self._server.register(login, role, connection)
+            # If authentication is a success, try to register the client:
+            try:
+                self._server.register(login, role, connection)
+            except AlreadyRegistered:
+                raise AuthenticationError('Authentication failure '
+                                          '(already connected)')
             connection.set_handler(WelcomeHandler.ROLES.get(role)(self._server))
             logging.info('New authentication from %s: success' % login)