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

[Jeremy Bicha <jbicha src gnome org>]

commit 7a33334c16eae193edf39d66663b44ce757ec4f2
Author: Jan Tojnar <jtojnar gmail com>
Date:   Sun Dec 17 18:41:53 2017 +0100

    appearance: Don't duplicate the cursor 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 |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/gtweak/tweaks/tweak_group_appearance.py b/gtweak/tweaks/tweak_group_appearance.py
index 0a73ecc..8227089 100644
--- a/gtweak/tweaks/tweak_group_appearance.py
+++ b/gtweak/tweaks/tweak_group_appearance.py
@@ -100,7 +100,7 @@ class CursorThemeSwitcher(GSettingsComboTweak):
         valid = walk_directories(dirs, lambda d:
                     os.path.isdir(d) and \
                         os.path.exists(os.path.join(d, "cursors")))
-        return valid
+        return set(valid)
 
 class ShellThemeTweak(Gtk.Box, Tweak):