REUTERS/Andy ClarkWASHINGTON, April 24 (Reuters) - U.S. Commerce Secretary Wilbur Ross said on Monday his agency will impose new anti-subsidy duties averaging 20 percent on Canadian softwood lumber imports, a move that escalates a long-running trade dispute between the two countries. Speaking to Reuters by phone a day ahead of the expected announcement, Ross said that the duties would affect $5 billion worth of softwood lumber imports from Canada. "It's about 31.5 percent of the total U.S.