[gnome-tweak-tool] appearance: Don't duplicate the theme name

[Jeremy Bicha <jbicha src gnome org>]

commit 06be203d06b74376519becc7677b17e7e26f9495
Author: Jeremy Bicha <jbicha ubuntu com>
Date:   Fri Nov 3 14:31:08 2017 -0400

    appearance: Don't duplicate the theme name
    
    A theme name should not be listed more than once
    
    https://bugzilla.gnome.org/show_bug.cgi?id=762789

 gtweak/tweaks/tweak_group_appearance.py |   13 ++++---------
 1 files changed, 4 insertions(+), 9 deletions(-)
---
diff --git a/gtweak/tweaks/tweak_group_appearance.py b/gtweak/tweaks/tweak_group_appearance.py
index be79891..84dd120 100644
--- a/gtweak/tweaks/tweak_group_appearance.py
+++ b/gtweak/tweaks/tweak_group_appearance.py
@@ -53,16 +53,11 @@ class GtkThemeSwitcher(GSettingsComboTweak):
         dirs = ( os.path.join(gtweak.DATA_DIR, "themes"),
                  os.path.join(GLib.get_user_data_dir(), "themes"),
                  os.path.join(os.path.expanduser("~"), ".themes"))
-        valid = walk_directories(dirs, lambda d:
+        valid = ['Adwaita', 'HighContrast']
+        valid += walk_directories(dirs, lambda d:
                     os.path.exists(os.path.join(d, "gtk-3.0", "gtk.css")) or \
                          os.path.exists(os.path.join(d, "gtk-3.{}".format(gtk_ver))))
-
-        if 'Adwaita' not in valid:
-            valid += ['Adwaita']
-        if 'HighContrast' not in valid:
-            valid += ['HighContrast']
-
-        return valid
+        return set(valid)
 
 class IconThemeSwitcher(GSettingsComboTweak):
     def __init__(self, **options):
@@ -80,7 +75,7 @@ class IconThemeSwitcher(GSettingsComboTweak):
         valid = walk_directories(dirs, lambda d:
                     os.path.isdir(d) and \
                        os.path.exists(os.path.join(d, "index.theme")))
-        return valid
+        return set(valid)
 
 class CursorThemeSwitcher(GSettingsComboTweak):
     def __init__(self, **options):