1 files changed, 33 insertions, 11 deletions

diff --git a/activesupport/lib/active_support/callbacks.rb b/activesupport/lib/active_support/callbacks.rb
index 893c2500d7..f0389d0a1c 100644
--- a/activesupport/lib/active_support/callbacks.rb
+++ b/activesupport/lib/active_support/callbacks.rb
@@ -132,6 +132,10 @@ module ActiveSupport
         @@_callback_sequence += 1
       end
 
+      def object_filter?
+        @_is_object_filter
+      end
+
       def matches?(_kind, _filter)
         if @_is_object_filter
           _filter_matches = @filter.to_s.start_with?(_method_name_for_object_filter(_kind, _filter, false))
@@ -337,6 +341,7 @@ module ActiveSupport
           :terminator => "false",
           :scope => [ :kind ]
         }.merge!(config)
+        @callbacks_hash = Hash.new { |h, k| h[k] = [] }
       end
 
       def compile
@@ -361,20 +366,37 @@ module ActiveSupport
 
       private
 
-      def append_one(callback)
-        remove_duplicates(callback)
-        push(callback)
-      end
+        def append_one(callback)
+          handle_duplicates(callback)
+          push(callback)
+        end
 
-      def prepend_one(callback)
-        remove_duplicates(callback)
-        unshift(callback)
-      end
+        def prepend_one(callback)
+          handle_duplicates(callback)
+          unshift(callback)
+        end
 
-      def remove_duplicates(callback)
-        delete_if { |c| callback.duplicates?(c) }
-      end
+        # We check to see if this callback already exists. If it does (i.e. if
+        # +callback.duplicates?(c)+ for any callback +c+ in the list of
+        # callbacks), then we delete the previously defined callback.
+        #
+        # We make use of the rep-invariant that only one callback exists which
+        # might match the new callback. The +@callbacks_hash+ is keyed on the
+        # +kind+ and +filter+ of the callback, the attributes used to check if
+        # two callbacks match.
+        def handle_duplicates(callback)
+          if callback.object_filter?
+            scan_and_remove_duplicates(callback)
+          else
+            hash_key = [callback.kind, callback.filter]
+            delete @callbacks_hash[hash_key] if @callbacks_hash[hash_key]
+            @callbacks_hash[hash_key] = callback
+          end
+        end
 
+        def scan_and_remove_duplicates(callback)
+          delete_if { |c| callback.duplicates?(c) }
+        end
     end
 
     module ClassMethods