diff --git a/History.rdoc b/History.rdoc
index bf1abef987..f44dc37d2b 100644
--- a/History.rdoc
+++ b/History.rdoc
@@ -94,6 +94,9 @@
     Andrea Singh.
   * The method parameter coverage report no longer includes parameter default
     values.  Issue #77 by Jake Goulding.
+  * The module for an include is not looked up until parsed all the files are
+    parsed.  Unless your project includes nonexistent modules this avoids
+    worst-case behavior (<tt>O(n!)</tt>) of RDoc::Include#module.
 
 === 3.9.2 / 2011-08-11
 
diff --git a/Rakefile b/Rakefile
index 9f074e3629..9a7fbee002 100644
--- a/Rakefile
+++ b/Rakefile
@@ -2,6 +2,8 @@ $:.unshift File.expand_path 'lib'
 require 'rdoc'
 require 'hoe'
 
+ENV['BENCHMARK'] = 'yes'
+
 task :docs    => :generate
 task :test    => :generate
 
diff --git a/lib/rdoc/class_module.rb b/lib/rdoc/class_module.rb
index 4e119837fa..52258b5bf1 100644
--- a/lib/rdoc/class_module.rb
+++ b/lib/rdoc/class_module.rb
@@ -374,6 +374,8 @@ def merge class_module
       end
     end
 
+    @includes.uniq! # clean up
+
     merge_collections method_list, cm.method_list, other_files do |add, meth|
       if add then
         add_method meth
@@ -609,6 +611,8 @@ def update_includes
       mod = include.module
       !(String === mod) && RDoc::TopLevel.all_modules_hash[mod.full_name].nil?
     end
+
+    includes.uniq!
   end
 
 end
diff --git a/lib/rdoc/context.rb b/lib/rdoc/context.rb
index e290ba4d45..3691fc60b3 100644
--- a/lib/rdoc/context.rb
+++ b/lib/rdoc/context.rb
@@ -398,8 +398,7 @@ def add_constant constant
   # Adds included module +include+ which should be an RDoc::Include
 
   def add_include include
-    add_to @includes, include unless
-      @includes.map { |i| i.full_name }.include? include.full_name
+    add_to @includes, include
 
     include
   end
diff --git a/lib/rdoc/include.rb b/lib/rdoc/include.rb
index c40fecf4dd..bbedce0472 100644
--- a/lib/rdoc/include.rb
+++ b/lib/rdoc/include.rb
@@ -15,7 +15,7 @@ def initialize(name, comment)
     super()
     @name = name
     self.comment = comment
-    @module = nil   # cache for module if found
+    @module = nil # cache for module if found
   end
 
   ##
@@ -28,10 +28,11 @@ def <=> other
   end
 
   def == other # :nodoc:
-    self.class == other.class and
-      self.name == other.name
+    self.class === other and @name == other.name
   end
 
+  alias eql? ==
+
   ##
   # Full name based on #module
 
@@ -40,6 +41,10 @@ def full_name
     RDoc::ClassModule === m ? m.full_name : @name
   end
 
+  def hash # :nodoc:
+    [@name, self.module].hash
+  end
+
   def inspect # :nodoc:
     "#<%s:0x%x %s.include %s>" % [
       self.class,
@@ -57,6 +62,13 @@ def inspect # :nodoc:
   # - if not found, look into the children of included modules,
   #   in reverse inclusion order;
   # - if still not found, go up the hierarchy of names.
+  #
+  # This method has <code>O(n!)</code> behavior when the module calling
+  # include is referencing nonexistent modules.  Avoid calling #module until
+  # after all the files are parsed.  This behavior is due to ruby's constant
+  # lookup behavior.
+  #
+  # As of the beginning of October, 2011, no gem includes nonexistent modules.
 
   def module
     return @module if @module
diff --git a/lib/rdoc/test_case.rb b/lib/rdoc/test_case.rb
index e5fdce2b90..3cf0eefe67 100644
--- a/lib/rdoc/test_case.rb
+++ b/lib/rdoc/test_case.rb
@@ -1,5 +1,6 @@
 require 'rubygems'
 require 'minitest/autorun'
+require 'minitest/benchmark' if ENV['BENCHMARK']
 
 require 'fileutils'
 require 'pp'
diff --git a/test/test_rdoc_class_module.rb b/test/test_rdoc_class_module.rb
index d931a22037..54caaec85a 100644
--- a/test/test_rdoc_class_module.rb
+++ b/test/test_rdoc_class_module.rb
@@ -774,6 +774,19 @@ def test_update_includes
     assert_equal [a, c], @c1.includes
   end
 
+  def test_update_includes_trim
+    a = RDoc::Include.new 'D::M', nil
+    b = RDoc::Include.new 'D::M', nil
+
+    @c1.add_include a
+    @c1.add_include b
+    @c1.ancestors # cache included modules
+
+    @c1.update_includes
+
+    assert_equal [a], @c1.includes
+  end
+
   def test_update_includes_with_colons
     a = RDoc::Include.new 'M1', nil
     b = RDoc::Include.new 'M1::M2', nil
diff --git a/test/test_rdoc_context.rb b/test/test_rdoc_context.rb
index 4dcac5a119..192a287b8c 100644
--- a/test/test_rdoc_context.rb
+++ b/test/test_rdoc_context.rb
@@ -183,16 +183,6 @@ def test_add_include
     assert_equal [incl], @context.includes
   end
 
-  def test_add_include_twice
-    incl1 = RDoc::Include.new 'Name', 'comment'
-    @context.add_include incl1
-
-    incl2 = RDoc::Include.new 'Name', 'comment'
-    @context.add_include incl2
-
-    assert_equal [incl1], @context.includes
-  end
-
   def test_add_method
     meth = RDoc::AnyMethod.new nil, 'old_name'
     meth.visibility = nil
@@ -334,6 +324,16 @@ def test_add_to_done_documenting
     refute_includes arr, incl
   end
 
+  def bench_add_include
+    cm = RDoc::ClassModule.new 'Klass'
+
+    assert_performance_linear 0.9 do |count|
+      count.times do |i|
+        cm.add_include RDoc::Include.new("N::M#{i}", nil)
+      end
+    end
+  end
+
   def test_child_name
     assert_equal 'C1::C1', @c1.child_name('C1')
   end