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Many cloud storage providers declare that they store multiple replicas of clients' data in order to prevent data loss. However, currently there is no guarantee that they actually spend storage for multiple replicas. Recently a multiple-replica provable data possession (MR-PDP) protocol is proposed, which provides clients with the ability to check whether multiple replicas are really stored at the cloud storage servers. However, in MR-PDP, only private verifiability is achieved. In this paper, we propose a multiple-replica remote data possession checking protocol which has public verifiability. The public verifiability increases the protocol's flexibility in that a third-party auditor can perform the data checking on behalf of the clients. Homomorphic authentication tags based on BLS signature are used in the proposed protocol. By security analysis and performance analysis, the proposed protocol is shown to be secure and efficient, which makes it very suitable in cloud storage systems.