diff --git a/plugins/Alias.py b/plugins/Alias.py
index c50dfd726..8849913be 100644
--- a/plugins/Alias.py
+++ b/plugins/Alias.py
@@ -217,9 +217,10 @@ class Alias(callbacks.Privmsg):
             raise AliasError, s
         name = realName
         cbs = callbacks.findCallbackForCommand(irc, name)
-        if [cb for cb in cbs if cb != self]:
-            s = 'A command with the name %r already exists.' % name
-            raise AliasError, s
+        if self in cbs:
+            if hasattr(self, realName) and realName not in self.aliases:
+                s = 'You can\'t overwrite commands in this plugin.'
+                raise AliasError, s
         if name in self.aliases:
             (currentAlias, locked) = self.aliases[name]
             if locked and currentAlias != alias:
diff --git a/test/test_Alias.py b/test/test_Alias.py
index 1f3956589..d1dd30477 100644
--- a/test/test_Alias.py
+++ b/test/test_Alias.py
@@ -67,6 +67,13 @@ class FunctionsTest(unittest.TestCase):
 
 class AliasTestCase(ChannelPluginTestCase, PluginDocumentation):
     plugins = ('Alias', 'Fun', 'Utilities')
+    def testDoesNotOverwriteCommands(self):
+        self.assertError('alias add alias "echo foo bar baz"')
+        self.assertError('alias add add "echo foo bar baz"')
+        self.assertError('alias add remove "echo foo bar baz"')
+        self.assertError('alias add lock "echo foo bar baz"')
+        self.assertError('alias add unlock "echo foo bar baz"')
+        
     def testAliasHelp(self):
         self.assertNotError('alias add slashdot foo')
         self.assertRegexp('help slashdot', "Alias for 'foo'")