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The stability of the redundant pumping scheme for an erbium-doped fiber amplifier is investigated. It is shown experimentally that an unstable operation occurs in the redundant pumping configuration and that the instability stems from the injection locking due to coupling between pump lasers. Two countermeasures for reducing the interaction between pump lasers are proposed for practical use-by using optical isolators and by using fiber gratings to obtain coherence collapse phenomenon. A theoretical model for the redundant pumping scheme is investigated, and the requirement for the isolation between the pump lasers to suppress the instability is verified.