duplicates=CustomField.connection.select_rows("SELECT custom_field_id, tracker_id FROM #{table_name} GROUP BY custom_field_id, tracker_id HAVING COUNT(*) > 1")
duplicates.eachdo|custom_field_id,tracker_id|
# Removes duplicate rows
CustomField.connection.execute("DELETE FROM #{table_name} WHERE custom_field_id=#{custom_field_id} AND tracker_id=#{tracker_id}")
# And insert one
CustomField.connection.execute("INSERT INTO #{table_name} (custom_field_id, tracker_id) VALUES (#{custom_field_id}, #{tracker_id})")