The federal government says Canada will challenge recent increases to softwood lumber duties by the United States.    International Trade Minister Mary Ng says Canada has taken steps to launch two legal challenges under the Canada-United States-Mexico Agreement. Ng describes the duties as unfair and unjust and says they will harm Canadian industry and increase housing costs.    In August, the U.S. Department of Commerce nearly doubled duties on softwood lumber. Its fifth administrative review of the duties increased the rate to just over 14.5 per cent from just over ...