If there were actually any such duty, the US long ago fulfilled it by investing trillions of dollars of capital into China and allowing dramatically unbalanced trade to go on for decades (resulting in trillions of dollars in trade deficits; not to mention the unfair dumping by state Chinese companies, countless economic regulations in China that are against the rules or spirit of the WTO, and so on).

The US built modern China in the exact same way the old European powers helped build up the US with vast capital inflows and trade.

All the parties have benefited. One can argue China benefited more, however the very undeveloped party in the transaction will usually benefit a lot more (again as was the case with the early US economic development in trade/investment with the British Empire or France).