diff --git a/src/conf.py b/src/conf.py
index a6f52852c..9bb4ee14b 100644
--- a/src/conf.py
+++ b/src/conf.py
@@ -573,8 +573,12 @@ utils.transactionalFile = transactionalFile
 
 class PluginDirectories(registry.CommaSeparatedListOfStrings):
     def __call__(self):
-        v = registry.CommaSeparatedListOfStrings.__call__(self)
-        return v + [_srcDir, _pluginsDir]
+        v = registry.CommaSeparatedListOfStrings.__call__(self)[:] # Copy!
+        if _srcDir not in v:
+            v.append(_srcDir)
+        if _pluginsDir not in v:
+            v.append(_pluginsDir)
+        return v
 
 registerGlobalValue(supybot.directories, 'plugins',
     PluginDirectories(['plugins'], """Determines what directories the bot will