diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index f83bbcdced74b..eca24cde1a210 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,5 +1,10 @@
 ## Rails 3.2.12 (unreleased) ##
 
+*   `BestStandardsSupport` no longer duplicates `X-UA-Compatible` values on
+    each request to prevent header size from blowing up.
+
+    *Edward Anderson*
+
 *   Fixed JSON params parsing regression for non-object JSON content.
 
     *Dylan Smith*
diff --git a/actionpack/lib/action_dispatch/middleware/best_standards_support.rb b/actionpack/lib/action_dispatch/middleware/best_standards_support.rb
index d338996240515..94efeb79fa5e3 100644
--- a/actionpack/lib/action_dispatch/middleware/best_standards_support.rb
+++ b/actionpack/lib/action_dispatch/middleware/best_standards_support.rb
@@ -17,7 +17,9 @@ def call(env)
       status, headers, body = @app.call(env)
 
       if headers["X-UA-Compatible"] && @header
-        headers["X-UA-Compatible"] << "," << @header.to_s
+        unless headers["X-UA-Compatible"][@header]
+          headers["X-UA-Compatible"] << "," << @header.to_s
+        end
       else
         headers["X-UA-Compatible"] = @header
       end
diff --git a/actionpack/test/dispatch/best_standards_support_test.rb b/actionpack/test/dispatch/best_standards_support_test.rb
index 0737c40a39547..551bb9621a3cc 100644
--- a/actionpack/test/dispatch/best_standards_support_test.rb
+++ b/actionpack/test/dispatch/best_standards_support_test.rb
@@ -16,9 +16,10 @@ def test_without_best_standards_support
     assert_equal nil, headers["X-UA-Compatible"]
   end
 
-  def test_appends_to_app_headers
+  def test_appends_to_app_headers_without_duplication_after_multiple_requests
     app_headers = { "X-UA-Compatible" => "requiresActiveX=true" }
     _, headers, _ = app(true, app_headers).call({})
+    _, headers, _ = app(true, app_headers).call({})
 
     expects = "requiresActiveX=true,IE=Edge,chrome=1"
     assert_equal expects, headers["X-UA-Compatible"]