diff --git a/actionpack/lib/action_dispatch/routing/route_set.rb b/actionpack/lib/action_dispatch/routing/route_set.rb
index aa0803e0e3ce06941574f7c297e15f4853cd9d6d..14c5d663a3fb672e6cadfcfc9c45897d5e836ef6 100644
--- a/actionpack/lib/action_dispatch/routing/route_set.rb
+++ b/actionpack/lib/action_dispatch/routing/route_set.rb
@@ -90,7 +90,7 @@ class NamedRouteCollection #:nodoc:
 
         def initialize
           @routes  = {}
-          @helpers = []
+          @helpers = Set.new
           @module  = Module.new
         end
 
@@ -100,7 +100,7 @@ def helper_names
 
         def clear!
           @helpers.each do |helper|
-            @module.remove_possible_method helper
+            @module.send :undef_method, helper
           end
 
           @routes.clear
@@ -108,7 +108,11 @@ def clear!
         end
 
         def add(name, route)
-          routes[name.to_sym] = route
+          key = name.to_sym
+          if routes.key? key
+            undef_named_route_methods @module, name
+          end
+          routes[key] = route
           define_named_route_methods(@module, name, route)
         end
 
@@ -256,7 +260,6 @@ def handle_positional_args(controller_options, inner_options, args, result, path
         def define_url_helper(mod, route, name, opts, route_key, url_strategy)
           helper = UrlHelper.create(route, opts, route_key, url_strategy)
 
-          mod.remove_possible_method name
           mod.module_eval do
             define_method(name) do |*args|
               options = nil
@@ -272,6 +275,11 @@ def define_named_route_methods(mod, name, route)
           define_url_helper mod, route, :"#{name}_path", route.defaults, name, PATH
           define_url_helper mod, route, :"#{name}_url", route.defaults, name, FULL
         end
+
+        def undef_named_route_methods(mod, name)
+          mod.send :undef_method, :"#{name}_path"
+          mod.send :undef_method, :"#{name}_url"
+        end
       end
 
       # :stopdoc: