China's trade surplus appears to have nearly doubled to US$20 billion from US$11.2 billion a year earlier in March according to economists, adding to friction that may prompt the US to file a World Trade Organization complaint. US lawmakers charge that China keeps its currency undervalued, protects piracy and subsidizes products sold overseas, Bloomberg reported. In March, the US Commerce Department levied duties on coated paper imports from China and the government may file a WTO complaint over the massive copyright infringement of movies and books.