*yasnippet-snippets.el: avoid duplicate loading the snippets (#468)

Author: sunlin7

yasnippet-snippets.el

@@ -49,8 +49,9 @@
   ;; NOTE: we add the symbol `yasnippet-snippets-dir' rather than its
   ;; value, so that yasnippet will automatically find the directory
   ;; after this package is updated (i.e., moves directory).
-  (add-to-list 'yas-snippet-dirs 'yasnippet-snippets-dir t)
-  (yas--load-snippet-dirs))
+  (unless (member 'yasnippet-snippets-dir yas-snippet-dirs)
+    (add-to-list 'yas-snippet-dirs 'yasnippet-snippets-dir t)
+    (yas--load-snippet-dirs)))
 
 (defgroup yasnippet-snippets nil
   "Options for yasnippet setups.