Disallow adding duplicate comments to a module

The C parser could add the same comment twice.  This change works around
this bug in the C parser.

Author: drbrain

lib/rdoc/class_module.rb

@@ -134,6 +134,11 @@ def add_comment comment, location
               else
                 normalize_comment comment
               end
+
+    return if @comment_location.any? do |(c, l)|
+      c == comment and l == location
+    end
+
     @comment_location << [comment, location]
 
     self.comment = original

test/test_rdoc_class_module.rb

@@ -41,6 +41,16 @@ def test_add_comment_comment
     assert_equal 'comment', cm.comment.text
   end
 
+  def test_add_comment_duplicate
+    tl1 = @store.add_file 'one.rb'
+
+    cm = RDoc::ClassModule.new 'Klass'
+    cm.add_comment '# comment 1', tl1
+    cm.add_comment '# comment 1', tl1
+
+    assert_equal [['comment 1', tl1]], cm.comment_location
+  end
+
   def test_add_comment_stopdoc
     tl = @store.add_file 'file.rb'