diff --git a/db/migrate/20110330175950_tag_uniqueness.rb b/db/migrate/20110330175950_tag_uniqueness.rb
new file mode 100644
index 000000000..bd4a4cd55
--- /dev/null
+++ b/db/migrate/20110330175950_tag_uniqueness.rb
@@ -0,0 +1,24 @@
+class TagUniqueness < ActiveRecord::Migration
+  def self.delete_duplicate_taggings
+    duplicate_rows = execute <<SQL
+      SELECT COUNT(t.taggable_id), t.taggable_id, t.taggable_type, t.tag_id FROM taggings AS t
+        GROUP BY t.taggable_id, t.taggable_type, t.tag_id
+          HAVING COUNT(*)>1;
+SQL
+    duplicate_rows.each do |row|
+      execute <<SQL
+        DELETE FROM taggings
+        WHERE taggings.taggable_id = #{row[1]} AND taggings.taggable_type = '#{row[2]}' AND taggings.tag_id = #{row[3]}
+        LIMIT #{row[0]-1}
+SQL
+    end
+  end
+  def self.up
+    delete_duplicate_taggings
+    add_index :taggings, [:taggable_id, :taggable_type, :tag_id], :unique => true, :name => 'index_taggings_uniquely'
+  end
+
+  def self.down
+    remove_index :taggings, :name => 'index_taggings_uniquely'
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index f199a9998..578cee0b6 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended to check this file into your version control system.
 
-ActiveRecord::Schema.define(:version => 20110328202414) do
+ActiveRecord::Schema.define(:version => 20110330175950) do
 
   create_table "aspect_memberships", :force => true do |t|
     t.integer  "aspect_id",  :null => false
@@ -329,6 +329,7 @@ ActiveRecord::Schema.define(:version => 20110328202414) do
 
   add_index "taggings", ["tag_id"], :name => "index_taggings_on_tag_id"
   add_index "taggings", ["taggable_id", "taggable_type", "context"], :name => "index_taggings_on_taggable_id_and_taggable_type_and_context"
+  add_index "taggings", ["taggable_id", "taggable_type", "tag_id"], :name => "index_taggings_uniquely", :unique => true
 
   create_table "tags", :force => true do |t|
     t.string "name"