Ensure the post categories are unique (only one). Ref: #2527 #2531

h/t @Nicholas-Perry
https://github.com/jekyll/jekyll/issues/2527#issuecomment-47041756

lib/jekyll/post.rb

@@ -78,8 +78,9 @@ module Jekyll
 
     def populate_categories
       categories_from_data = Utils.pluralized_array_from_hash(data, 'category', 'categories')
-      self.categories = (Array(categories) + categories_from_data).map {|c| c.to_s.downcase}
-      categories.flatten!
+      self.categories = (
+        Array(categories) + categories_from_data
+      ).map {|c| c.to_s.downcase}.flatten.uniq
     end
 
     def populate_tags