For the second time in five years, a federal court has upheld Amgen Inc.'s patent on its blockbuster biotechnology drug Epogen, after a challenge was filed by rival Genetics Institute. The dispute involved Epogen, Amgen's drug for anemia, which generated $883 million in sales last year for the Thousand Oaks company. In 1991, a federal court ruled that Amgen's patent covered the key biotechnology process for producing Epogen, and it said that Genetics Institute's patent was invalid. But in 1994, Genetics Institute received another Epogen-like patent. On Friday a U.S. Court of Appeals ruled that Amgen's existing patent barred Genetics Institute from making its own version of the drug.