diff --git a/db/migrate/20130207181455_add_unique_index_on_custom_fields_projects.rb b/db/migrate/20130207181455_add_unique_index_on_custom_fields_projects.rb
new file mode 100644
index 000000000..223818d4f
--- /dev/null
+++ b/db/migrate/20130207181455_add_unique_index_on_custom_fields_projects.rb
@@ -0,0 +1,24 @@
+class AddUniqueIndexOnCustomFieldsProjects < ActiveRecord::Migration
+  def up
+    table_name = "#{CustomField.table_name_prefix}custom_fields_projects#{CustomField.table_name_suffix}"
+    duplicates = CustomField.connection.select_rows("SELECT custom_field_id, project_id FROM #{table_name} GROUP BY custom_field_id, project_id HAVING COUNT(*) > 1")
+    duplicates.each do |custom_field_id, project_id|
+      # Removes duplicate rows
+      CustomField.connection.execute("DELETE FROM #{table_name} WHERE custom_field_id=#{custom_field_id} AND project_id=#{project_id}")
+      # And insert one
+      CustomField.connection.execute("INSERT INTO #{table_name} (custom_field_id, project_id) VALUES (#{custom_field_id}, #{project_id})")
+    end
+
+    if index_exists? :custom_fields_projects, [:custom_field_id, :project_id]
+      remove_index :custom_fields_projects, [:custom_field_id, :project_id]
+    end
+    add_index :custom_fields_projects, [:custom_field_id, :project_id], :unique => true
+  end
+
+  def down
+    if index_exists? :custom_fields_projects, [:custom_field_id, :project_id]
+      remove_index :custom_fields_projects, [:custom_field_id, :project_id]
+    end
+    add_index :custom_fields_projects, [:custom_field_id, :project_id]
+  end
+end