[pitivi] effects: Fix adding an element to a set

[Thibault Saunier <tsaunier src gnome org>]

commit ad6d68544fb6118d1087c112d3c9f59c15f85047
Author: Alexandru Băluț <alexandru balut gmail com>
Date:   Tue Sep 30 00:14:22 2014 +0200

    effects: Fix adding an element to a set

 pitivi/effects.py |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/pitivi/effects.py b/pitivi/effects.py
index aaf1650..47e2adc 100644
--- a/pitivi/effects.py
+++ b/pitivi/effects.py
@@ -242,7 +242,7 @@ class EffectsManager(object):
             for name in category[1]:
                 if name == effect_name:
                     categories.append(category[0])
-                    self._audio_categories.append(category[0])
+                    self._audio_categories.add(category[0])
 
         for category in self._video_categories_effects:
             for name in category[1]: