diff --git a/CHANGELOG b/CHANGELOG
index e333dafae30..b819fe8b830 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,6 @@
 = ActiveMerchant CHANGELOG
 
+* Add support for Authorize.net duplicate window [Seamus Abshere]
 * Return transaction id for PayPal refunds [jxtps435]
 * Allow storage of e-checks with BraintreeGateway [jimiray]
 * Add test URL to PayJunction gateway [boomtowndesigngroup]
diff --git a/lib/active_merchant/billing/gateways/authorize_net.rb b/lib/active_merchant/billing/gateways/authorize_net.rb
index 2ae4ebd389c..f790fdcd14c 100644
--- a/lib/active_merchant/billing/gateways/authorize_net.rb
+++ b/lib/active_merchant/billing/gateways/authorize_net.rb
@@ -33,6 +33,8 @@ class AuthorizeNetGateway < Gateway
 
       self.arb_test_url = 'https://apitest.authorize.net/xml/v1/request.api'
       self.arb_live_url = 'https://api.authorize.net/xml/v1/request.api'
+      
+      class_inheritable_accessor :duplicate_window
 
       APPROVED, DECLINED, ERROR, FRAUD_REVIEW = 1, 2, 3, 4
 
@@ -86,6 +88,7 @@ def authorize(money, creditcard, options = {})
         add_creditcard(post, creditcard)
         add_address(post, options)
         add_customer_data(post, options)
+        add_duplicate_window(post)
 
         commit('AUTH_ONLY', money, post)
       end
@@ -103,6 +106,7 @@ def purchase(money, creditcard, options = {})
         add_creditcard(post, creditcard)
         add_address(post, options)
         add_customer_data(post, options)
+        add_duplicate_window(post)
 
         commit('AUTH_CAPTURE', money, post)
       end
@@ -319,6 +323,15 @@ def add_customer_data(post, options)
           post[:customer_ip] = options[:ip]
         end
       end
+      
+      # x_duplicate_window won't be sent by default, because sending it changes the response.
+      # "If this field is present in the request with or without a value, an enhanced duplicate transaction response will be sent."
+      # (as of 2008-12-30) http://www.authorize.net/support/AIM_guide_SCC.pdf
+      def add_duplicate_window(post)
+        unless duplicate_window.nil?
+          post[:duplicate_window] = duplicate_window
+        end
+      end
 
       def add_address(post, options)
 
diff --git a/test/unit/gateways/authorize_net_test.rb b/test/unit/gateways/authorize_net_test.rb
index 0b6a81bbfab..85c9947eb78 100644
--- a/test/unit/gateways/authorize_net_test.rb
+++ b/test/unit/gateways/authorize_net_test.rb
@@ -73,6 +73,20 @@ def test_add_description
     assert_equal 'My Purchase is great', result[:description]
   end
   
+  def test_add_duplicate_window_without_duplicate_window
+    result = {}
+    ActiveMerchant::Billing::AuthorizeNetGateway.duplicate_window = nil
+    @gateway.send(:add_duplicate_window, result)
+    assert_nil result[:duplicate_window]
+  end
+  
+  def test_add_duplicate_window_with_duplicate_window
+    result = {}
+    ActiveMerchant::Billing::AuthorizeNetGateway.duplicate_window = 0
+    @gateway.send(:add_duplicate_window, result)
+    assert_equal 0, result[:duplicate_window]
+  end
+  
   def test_purchase_is_valid_csv
    params = { :amount => '1.01' }