diff --git a/lib/jekyll/post.rb b/lib/jekyll/post.rb
index b1496de3..4afe82e6 100644
--- a/lib/jekyll/post.rb
+++ b/lib/jekyll/post.rb
@@ -218,7 +218,7 @@ module Jekyll
         :title       => slug,
         :i_day       => date.strftime("%-d"),
         :i_month     => date.strftime("%-m"),
-        :categories  => (categories || []).map { |c| c.to_s.downcase }.join('/'),
+        :categories  => (categories || []).map { |c| c.to_s.downcase }.uniq.join('/'),
         :short_month => date.strftime("%b"),
         :short_year  => date.strftime("%y"),
         :y_day       => date.strftime("%j"),
diff --git a/test/test_post.rb b/test/test_post.rb
index 44cc9664..0e6861ce 100644
--- a/test/test_post.rb
+++ b/test/test_post.rb
@@ -276,6 +276,19 @@ class TestPost < Test::Unit::TestCase
           end
         end
 
+        context "with duplicated mixed case (categories)" do
+          setup do
+            @post.categories << "MixedCase"
+            @post.categories << "Mixedcase"
+            @post.process(@fake_file)
+          end
+
+          should "process the url correctly" do
+            assert_equal "/:categories/:year/:month/:day/:title.html", @post.template
+            assert_equal "/mixedcase/2008/09/09/foo-bar.html", @post.url
+          end
+        end
+
         context "with none style" do
           setup do
             @post.site.permalink_style = :none