According to TFA, Novartis, the inventor of (non-generic) Diovan, found the NDMA when they bought some (generic) Valsartan from Huahai. They were the first customer to test thoroughly enough to find it. So, contamination in non-generics is possible, but probably less likely. Actually the NDMA wasn't produced in the original manufacturing process designated for Diovan and Valsartan, but Huahai used a different process to save money.