game.io.send('CHAT ' + quote(t.nickname + ': ' + msg))
 cmd_ALL.argtypes = 'string'
 
-# TOOD split into two commands
 def cmd_LOGIN(game, nick, connection_id):
     for t in [t for t in game.things if t.type_ == 'player' and t.nickname == nick]:
         raise GameError('name already in use')
     if connection_id in game.sessions:
-        t_id = game.sessions[connection_id]
-        t = game.get_thing(t_id, False)
-        old_nick = t.nickname
-        t.nickname = nick
-        game.io.send('CHAT ' + quote(old_nick + ' renamed themselves to ' + nick))
-    else:
-        t = game.thing_types['player'](game)
-        t.position = YX(game.map.size.y // 2, game.map.size.x // 2)
-        game.things += [t]  # TODO refactor into Thing.__init__?
-        game.sessions[connection_id] = t.id_
-        game.io.send('LOGIN_OK', connection_id)
-        t.nickname = nick
-        game.io.send('CHAT ' + quote(t.nickname + ' entered the map.'))
-        game.io.send('PLAYER_ID %s' % t.id_, connection_id)
+        raise GameError('cannot log in twice')
+    t = game.thing_types['player'](game)
+    t.position = YX(game.map.size.y // 2, game.map.size.x // 2)
+    game.things += [t]  # TODO refactor into Thing.__init__?
+    game.sessions[connection_id] = t.id_
+    game.io.send('LOGIN_OK', connection_id)
+    t.nickname = nick
+    game.io.send('CHAT ' + quote(t.nickname + ' entered the map.'))
+    game.io.send('PLAYER_ID %s' % t.id_, connection_id)
     game.changed = True
 cmd_LOGIN.argtypes = 'string'
 
+def cmd_NICK(game, nick, connection_id):
+    for t in [t for t in game.things if t.type_ == 'player' and t.nickname == nick]:
+        raise GameError('name already in use')
+    if not connection_id in game.sessions:
+        raise GameError('can only rename when already logged in')
+    t_id = game.sessions[connection_id]
+    t = game.get_thing(t_id, False)
+    old_nick = t.nickname
+    t.nickname = nick
+    game.io.send('CHAT ' + quote(old_nick + ' renamed themselves to ' + nick))
+    game.changed = True
+cmd_NICK.argtypes = 'string'
+
 def cmd_GET_GAMESTATE(game, connection_id):
     game.send_gamestate(connection_id)
 cmd_GET_GAMESTATE.argtypes = ''
 
 from plomrogue.game import Game
 from plomrogue.io_websocket import PlomWebSocketServer
 from plomrogue.io_tcp import PlomTCPServer
-from plomrogue.commands import (cmd_ALL, cmd_LOGIN, cmd_QUERY, cmd_PING, cmd_MAP,
-                                cmd_TURN, cmd_MAP_LINE, cmd_GET_ANNOTATION,
+from plomrogue.commands import (cmd_ALL, cmd_LOGIN, cmd_NICK, cmd_QUERY, cmd_PING,
+                                cmd_MAP, cmd_TURN, cmd_MAP_LINE, cmd_GET_ANNOTATION,
                                 cmd_ANNOTATE, cmd_PORTAL, cmd_GET_GAMESTATE)
 from plomrogue.tasks import (Task_WAIT, Task_MOVE, Task_WRITE,
                              Task_FLATTEN_SURROUNDINGS)
 game = Game(savefile)
 game.register_command(cmd_PING)
 game.register_command(cmd_LOGIN)
+game.register_command(cmd_NICK)
 game.register_command(cmd_QUERY)
 game.register_command(cmd_TURN)
 game.register_command(cmd_MAP)