From 6e104c3ed0c080edb2fef8ecd418de2972a48882 Mon Sep 17 00:00:00 2001
From: Parker Moore <parkrmoore@gmail.com>
Date: Tue, 24 Jun 2014 19:15:02 -0400
Subject: [PATCH] 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 | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib/jekyll/post.rb b/lib/jekyll/post.rb
index 6cd6e938..f13d8b2c 100644
--- a/lib/jekyll/post.rb
+++ b/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