diff --git a/lib/active_model/serializer/associations.rb b/lib/active_model/serializer/associations.rb
index c4da3515..42e872ce 100644
--- a/lib/active_model/serializer/associations.rb
+++ b/lib/active_model/serializer/associations.rb
@@ -76,8 +76,6 @@ module ActiveModel
         # @api private
         #
         def associate(reflection)
-          self._reflections = _reflections.dup
-
           self._reflections << reflection
         end
       end