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

[Jeremy Bicha <jbicha src gnome org>]

commit f1cd8d47f7a21409235dacbb8714b378bcdad5fa
Author: Jeremy Bicha <jbicha ubuntu com>
Date:   Sat Nov 4 12:31:59 2017 -0400

    appearance: Don't duplicate the GNOME Shell theme name
    
    https://bugzilla.gnome.org/show_bug.cgi?id=762789

 gtweak/tweaks/tweak_group_appearance.py |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/gtweak/tweaks/tweak_group_appearance.py b/gtweak/tweaks/tweak_group_appearance.py
index 84dd120..4358f37 100644
--- a/gtweak/tweaks/tweak_group_appearance.py
+++ b/gtweak/tweaks/tweak_group_appearance.py
@@ -164,6 +164,7 @@ class ShellThemeTweak(Gtk.Box, Tweak):
                         os.path.exists(os.path.join(d, "gnome-shell", "gnome-shell.css")))
             #the default value to reset the shell is an empty string
             valid.extend( ("",) )
+            valid = set(valid)
 
             #build a combo box with all the valid theme options
             #manually add Adwaita to represent the default