Is it normal for fuel injectors to go bad, one after another in about 3 weeks?

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Took the car to mechanic about 3 weeks with knock sensor and injector error codes. He replace #2 injector (stuck open). About a week or so later, same error codes and bad miss reappeared. Mechanic replaced #1 injector. A week later got car back and within less than 10 miles it all started again. What is causing these injectors to go bad one after another???